Enigma

1

THE NIGHT WAS impenetrable, the cold irresistible. Huddled in his overcoat inside the icy Rover, Tom Jericho could barely see the flickering of his breath or the mist it formed on the window beside him. He reached across and rubbed a porthole in the condensation, smearing his fingers with cold, wet grime. Occasionally their headlamps flashed on whitewashed cottages and darkened inns, and once they passed a convoy of lorries heading in the opposite direction. But mostly they seemed to travel in a void. There were no street lights or signposts to guide them, no lit windows; not even a match glimmered in the blackness. They might have been the last three people alive.

Logie had started to snore within fifteen minutes of leaving King's, his head dropping further forwards onto his chest each time the Rover hit a bump, a motion which caused him to mumble and nod, as if in profound agreement with himself. Once, when they turned a corner sharply, his long body toppled sideways and Jericho had to fend him off gently with his forearm.

In the front seat Leveret hadn't uttered a word, except to say, when Jericho asked him to turn it on, that the heater was broken. He was driving with exaggerated care, his face hunched inches from the windscreen, his right foot alternating cautiously between the brake pedal and the accelerator. At times they seemed to be travelling scarcely faster than walking-pace, so that although in daylight the journey to Bletchley might take little more than an hour and a half, Jericho calculated that tonight they would be lucky to reach their destination before midnight.

'I should get some sleep if I were you, old thing,' Logie had said, making a pillow of his overcoat. 'Long night ahead.'

But Jericho couldn't sleep. He stuffed his hands deep into his pockets and stared uselessly into the night.

Bletchley, he thought with disgust. Even the sensation of the name in the mouth was unpleasant, stranded somewhere between blanching and retching. Of all the towns in England, why did they have to choose Bletchley? Four years ago he'd never even heard of the place. And he might have lived the rest of his life in happy ignorance had it not been for that glass of sherry in Atwood's rooms in the spring of 1939.

How odd it was, how absurd to trace one's destiny and to find that it revolved around a couple of fluid ounces of pale manzanilla.

It was immediately after that first approach that Atwood had arranged for him to meet some 'friends' in London. Thereafter, every Friday morning for four months, Jericho would catch an early train and make his way to a dusty office block near St James's tube station. Here, in a shabby room furnished by a blackboard and a clerk's desk, he was initiated into the secrets of cryptography. And it was just as Turing had predicted: he loved it.

He loved the history, all of it, from the ancient runic systems and the Irish codes of the Book of Ballymote with their exotic names ('Serpent through the heather', 'Vexation of a poet's heart'), through the codes of Pope Sylvester II and Hildegard von Bingen, through the invention of Alberti's cipher disk – the first poly-alphabetic cipher – and Cardinal Richelieu's grilles, all the way down to the machine-generated mysteries of the German Enigma, which were gloomily held to be unbreakable.

And he loved the secret vocabulary of cryptanalysis, with its homophones and polyphones, its digraphs and bigraphs and nulls. He studied frequency analysis. He was taught the intricacies of superencipherment, of placode and enicode. At the beginning of August 1939 he was formally offered a post at the Government Code and Cipher School at a salary of three hundred pounds a year and was told to go back to Cambridge and await developments. On 1 September he woke to hear on the wireless that the Germans had invaded Poland. On 3 September, the day Britain declared war, a telegram arrived at the Porter's Lodge ordering him to report the following morning to a place called Bletchley Park.

He left King's as instructed, as soon as it was light, wedged into the passenger seat of Atwood's antiquated sports car. Bletchley turned out to be a small Victorian railway town about fifty miles west of Cambridge. Atwood, who liked to cut a dash, insisted on driving with the roof off, and as they rattled down the narrow streets Jericho had an impression of smoke and soot, of little, ugly terraced houses and the tall, black chimneys of brick kilns. They passed under a railway bridge, along a lane, and were waved through a pair of high gates by armed sentries. To their right, a lawn sloped down to a lake fringed by large trees. To their left was a mansion – a long, low, late-Victorian monstrosity of red brick and sand-coloured stone that reminded Jericho of the veterans' hospital his father had died in. He looked around, half expecting to see wimpled nurses wheeling broken men in Bath chairs.

'Isn't it perfectly hideous?' squeaked Atwood with delight. 'Built by a Jew. A stockbroker. A friend of Lloyd George.' His voice rose with each statement, suggesting an ascending scale of social horror. He parked abruptly at a crazy angle, with a spurt of gravel, narrowly missing a sapper unrolling a large drum of electrical cable.

Inside, in a panelled drawing room overlooking the lake, sixteen men stood around drinking coffee. Jericho was surprised at how many he recognised. They glanced at one another, embarrassed and amused. So, their faces said, they got you too. Atwood moved serenely among them, shaking hands and making sharp remarks they all felt obliged to smile at.

'It's not fighting the Germans I object to. It's going to war on behalf of these beastly Poles.' He turned to a handsome, intense-looking young man with a broad, high forehead and thick hair. 'And what's your name?'

'Pukowski,' said the young man, in perfect English. 'I'm a beastly Pole.'

Turing caught Jericho's eye and winked.

In the afternoon the cryptanalysts were split into teams. Turing was assigned to work with Pukowski, redesigning the 'bombe', the giant decryptor which the great Marian Rejewski of the Polish Cipher Bureau had built in 1938 to attack Enigma. Jericho was sent to the stable block behind the mansion to analyse encrypted German radio traffic.

How odd they were, those first nine months of the war, how unreal, how – it seemed absurd to say it now – peaceful. They cycled in each day from their digs in various country pubs and guesthouses around the town. They lunched and dined together in the mansion. In the evenings they played chess and strolled through the grounds before cycling home to bed. There was even a Victorian maze of yew hedges to get lost in. Every ten days or so, someone new would join the party – a classicist, a mathematician, a museum curator, a dealer in rare books – each recruited because he was a friend of someone already resident in Bletchley.

A dry and smoky autumn of gold's and browns, the rooks whirling in the sky like cinders, gave way to a winter off a Christmas card. The lake froze. The elms drooped under the weight of snow. A robin pecked at breadcrumbs outside the stable window.

Jericho's work was pleasantly academic. Three or four times a day, a motorcycle dispatch-rider would clatter into the courtyard at the back of the big house bearing a pouch of intercepted German cryptograms. Jericho sorted them by frequency and call sign and marked them up on charts in coloured crayons – red for the Luftwaffe, green for the German Army – until gradually, from the unintelligible babble, shapes emerged. Stations in a radio net allowed to talk freely to one another made, when plotted on the stable wall, a crisscross pattern within a circle. Nets in which the only line of communication was two-way, between a headquarters and its out-stations, resembled stars. Circle-nets and star-nets. Kreis und Stern.

This idyll lasted eight months, until the German offensive in May 1940. Up to then, there had been scarcely enough material for the cryptanalysts to make a serious attack on Enigma. But as the Wehrmacht swept through Holland, Belgium, France, the babble of wireless traffic became a roar. From three or four motorcycle pouches of material, the volume increased to thirty or forty; to a hundred; to two hundred.

It was late one morning about a week after this had started that Jericho felt a touch on his elbow and turned to find Turing, smiling.

'There's someone I want you to meet, Tom.'

'I'm rather busy at present, Alan, to be honest.'

'Her name's Agnes. I really think you ought to see her.'

Jericho almost argued. A year later he would have argued, but at that time he was still too much in awe of Turing not to do as he was told. He tugged his jacket off the back of his chair and walked out, shrugging it on, into the May sunshine.

By this time the Park had already started to be transformed. Most of the trees at the side of the lake had been chopped down to make way for a series of large wooden huts. The maze had been uprooted and replaced by a low brick building, outside which a small crowd of cryptanalysts had gathered. There was a sound coming from within it, of a sort Jericho had never heard before, a humming and a clattering, something between a loom and a printing press. He followed Turing through the door. Inside, the noise was deafening, reverberating off the whitewashed walls and the corrugated iron ceiling. A brigadier, an air commodore, two men in overalls and a frightened-looking Wren with her fingers in her ears were standing round the edge of the room staring at a large machine full of revolving drums. A blue flash of electricity arced across the top. There was a fizz and a crackle, a smell of hot oil and overheated metal.

'It's the redesigned Polish bombe,' said Turing. 'I thought I'd call her Agnes.' He rested his long, pale fingers tenderly on the metal frame. There was a bang and he snatched them away again. 'I do hope she works all right

Oh yes, thought Jericho, rubbing another window into the condensation, oh yes, she worked all right.

The moon slid from beneath a cloud, briefly lighting the Great North Road. He closed his eyes.

She worked all right, and after that the world was different.

Despite his earlier wakefulness Jericho must have fallen asleep, for when he next opened his eyes Logie was sitting up and the Rover was passing through a small town. It was still dark and at first he couldn't get his bearings. But then they passed a row of shops, and when the headlights flickered briefly on the billboard of the County Cinema (NOW SHOWING: 'THE NAVY COMES THROUGH', 'SOMEWHERE I'LL FIND YOU'), he muttered to himself, and heard the weariness already creeping back into his voice: 'Bletchley.'

'Too bloody right,' said Logie.

Down Victoria Road, past the council offices, past a school… The road curved and suddenly, in the distance, above the pavements, a myriad of fireflies were swarming towards them. Jericho passed his hands across his face and found that his fingers were numb. He felt mildly sick.

'What time is it?'

'Midnight,' said Logie. 'Shift change.'

The specks of light were blackout torches.

Jericho guessed the Park's workforce must now be about five or six thousand, toiling round the clock in eight-hour shifts – midnight till eight, eight till four, four till midnight. That meant maybe four thousand people were now on the move, half coming off shift, half going on, and by the time the Rover had turned into the road leading to the main gate it was barely possible to advance a yard without hitting someone. Leveret was alternately leaning out of the window, shouting and hammering on the horn. Crowds of people had spilled out into the road, most on foot, some on bicycles. A convoy of buses was struggling to get past. Jericho thought: the odds are two to one that Claire's among them. He had a sudden desire to shrink down in his seat, to cover his head, to get away.

Logie was looking at him curiously. 'Are you sure you're up to this, old thing?'

I'm fine. It's just – it's hard to think it started with sixteen of us.'

'Wonderful, isn't it? And it'll be twice the size next year.' The pride in Logie's voice abruptly gave way to alarm. 'For God's sake, Leveret, look out man, you nearly ran that lady over!'

In the headlights a blonde head spun angrily and Jericho felt a rush of nausea. But it wasn't her. It was a woman he didn't recognise, a woman in an army uniform, a slash of scarlet lipstick like a wound across her face. She looked as if she was tarted up and on her way to meet a man. She shook her fist and mouthed 'Bugger off at them.

'Well,' said Logie, primly, 'I thought she was a lady.'

When they reached the guard post they had to dig out their identity cards. Leveret collected them and passed them on through the window to an RAF corporal. The sentry hitched his rifle and studied the cards by torchlight, then ducked down and directed the beam in turn on to each of their faces. The brilliance struck Jericho like a blow. Behind them he could hear a second sentry rummaging through the boot.

He flinched from the light and turned to Logie. 'When did all this start?' He could remember a time when they weren't even asked for passes.

'Not sure now you mention it.' Logie shrugged. 'They seem to have tightened up in the last week or two.'

Their cards were returned. The barrier rose. The sentry waved them through. Beside the road was a freshly painted sign. They had been given a new name some time around Christmas and Jericho could just about read the white lettering in the darkness: 'Government Communication Headquarters'.

The metal barrier came down after them with a crash.

2

Even in the blackout you could sense the size of the place. The mansion was still the same, and so were the huts, but these were now just a fraction of the overall site. Stretching away beyond them was a great factory of intelligence: low, brick-built offices and bombproof bunkers of concrete and steel, A-Blocks and B-Blocks and C-Blocks, tunnels and shelters and guard posts and garages… There was a big military camp just beyond the wire. The barrels of anti-aircraft batteries poked through camouflaged netting in the nearby woods. And more buildings were under construction. There had never been a day when Jericho hadn't heard the racket of mechanical diggers and cement mixers, the ringing of pickaxes and the splintering of falling trees. Once, just before he left, he had paced out the distance from the new assembly hall to the far perimeter fence and had reckoned it at half a mile. What was it all for? He had no idea. Sometimes he thought they must be monitoring every radio transmission on the planet.

Leveret drove the Rover slowly past the darkened mansion, past the tennis court and the generators, and drew up a short distance from the huts.

Jericho clambered stiffly from the back seat. His legs had gone to sleep and the sensation of the blood returning made his knees buckle. He leaned against the side of the car. His right shoulder was rigid with cold. A duck splashed somewhere on the lake and its cry made him think of Cambridge – of his warm bed and his crosswords – and he had to shake his head to clear the memory.

Logie was explaining to him that he had a choice: Leveret could take him over to his new digs and he could have a decent night's kip, or he could come in straight away and take a look at things immediately.

'Why don't we start now?' said Jericho. His re-entry into the hut would be an ordeal. He'd prefer to get it over with.

'That's the spirit, old love. Leveret will look after your cases, won't you, Leveret? And take them to Mr Jericho's room?'

'Yes, sir.' Leveret looked at Jericho for a moment, then stuck out his hand. 'Good luck, sir.'

Jericho took it. The solemnity surprised him. Anyone would think he was about to make a parachute jump into hostile territory. He tried to think of something to say. 'Thank you very much for driving us.'

Logie was fiddling with Leveret's blackout torch. 'What the hell's wrong with this thing?' He knocked it against his palm. 'Bloody thing. Oh, sod it. Come on.'

He strode away on his long legs and after a moment's hesitation Jericho wrapped his scarf tight around his neck and followed. In the darkness they had to feel their way along the blastproof wall surrounding Hut 8. Logie banged into what sounded like a bicycle and Jericho heard him swear. He dropped the torch. The impact made it come on. A trickle of light revealed the entrance to the hut. There was a smell of lime and damp here – lime and damp and creosote: the odours of Jericho's war. Logie rattled the handle, the door opened and they stepped into the dim glow.

Because he had changed so much in the month he had been away, somehow – illogically – he had expected that the hut would have changed as well. Instead, the instant he crossed the threshold, the familiarity of it almost overwhelmed him. It was like a recurrent dream in which the horror lay in knowing precisely what would happen next – the certainty that it always had been, and always would be, exactly like this.

A narrow, ill-lit corridor, perhaps twenty yards long, stretched in front of him, with a dozen doors leading off it. The wooden partitions were flimsy and the noise of a hundred people working at full stretch leaked from room to room – the clump and thud of boots and shoes on the bare boards, the hum of conversation, the occasional shout, the scrape of chair legs, the ringing of telephones, the clack clack clack of the Type-X machines in the Decoding Room.

The only tiny difference was that the walk-in cupboard on the right, immediately next to the entrance, now had a nameplate on it: 'Lt. Kramer US Navy Liaison Officer'.

Familiar faces loomed towards him. Kingcome and Proudfoot were whispering together outside the Catalogue Room and drew back to let him pass. He nodded to them. They nodded in return but didn't speak. Atwood hurried out of the Crib Room, saw Jericho, gawped, then put his head down. He muttered, 'Hello, Tom,' then almost ran towards Research.

Clearly, nobody had ever expected to see him again. He was an embarrassment. A dead man. A ghost.

Logie was oblivious, both to the general astonishment and to Jericho's discomfort. 'Hello, everybody.' He waved to Atwood. 'Hello, Frank. Look who's back! The prodigal returns! Give them a smile, Tom, old thing, it's not a ruddy funeral. Not yet, anyway.' He stopped outside his office and fiddled with his key for half a minute, then discovered the door was unlocked. 'Come in, come in.'

The room was scarcely bigger than a broom store. It had been Turing's cubbyhole until just before the break into Shark, when Turing had been sent to America. Now Logie had it – his tiny perquisite of rank – and he looked absurdly huge as he bent over his desk, like an adult poking around in a child's den. There was a fireproof safe in one corner, leaking intercepts, and a rubbish bin labelled CONFIDENTIAL WASTE. There was a telephone with a red handset. Paper was everywhere – on the floor, on the table, on the top of the radiator where it had baked crisp and yellow, in wire baskets and in box files, in tall stacks and in piles that had subsided into fans.

'Bugger, bugger, bugger.' Logie had a message slip in his hands and was frowning at it. He took his pipe out of his pocket and chewed on the stem. He seemed to have forgotten Jericho's presence until Jericho coughed to remind him.

'What? Oh. Sorry, old love.' He traced the words of the message with his pipe. 'The Admiralty's a bit exercised, apparently. Conference in A-Block at eight o'clock with Navy brass up from Whitehall. Want to know the score. Skynner's in a spin and demands to see me forthwith. Bugger, bugger.'

'Does Skynner know I'm back?' Skynner was the head of Bletchley's Naval Section. He'd never cared for Jericho, probably because Jericho had never concealed his opinion of him: that he was a bombast and a bully whose chief war aim was to greet the peace as Sir Leonard Skynner, OBE, with a seat on the Security Executive and a lease on an Oxford mastership. Jericho had a vague memory of actually telling Skynner some of this, or all of it, or possibly more, shortly before he was sent back to Cambridge to recover his senses.

'Of course he knows you're back, old thing. I had to clear it with him first.'

'And he doesn't mind?'

'Mind? No. 'The man's desperate. He'd do anything to get back into Shark.' Logie added quickly: 'Sorry, I don't mean… that's not to say that bringing you back is an act of desperation. Only, well, you know…' He sat down heavily and looked again at the message. He rattled his pipe against his worn yellow teeth. 'Bugger, bugger, bugger'

Looking at him then it occurred to Jericho that he knew almost nothing about Logie. They had worked together for two years, would regard themselves as friends, yet they'd never had a proper conversation. He didn't know if Logie was married, or if he had a girl.

'I'd better go and see him, I suppose. Excuse me, old love.'

Logie squeezed past his desk and shouted down the corridor: 'Puck!' Jericho could hear the cry being taken up somewhere in the recesses of the hut by another voice. 'Puck!' And then another: 'Puck! Puck!'

Logie ducked his head back into the office. 'One analyst per shift co-ordinates the Shark attack. Puck this shift, Baxter next, then Pettifer.' His head disappeared again. 'Ah-ha, here he comes. Come on, old thing. Look alive. I've a surprise for you. See who's in here.'

'So there you are, my dear Guy,' came a familiar voice from the corridor. 'Nobody knew where to find you.'

Adam Pukowski slid his lithe frame past Logie, saw Jericho and stopped dead. He was genuinely shocked. Jericho could almost see his mind struggling to regain control of his features, forcing his famous smile back on to his face. At last he managed it. He even threw his arms round Jericho and hugged him. 'Tom, it's… I had begun to think you were never returning. It's marvellous.'

'It's good to see you again, Puck.' Jericho patted him politely on the back.

Puck was their mascot, their touch of glamour, their link with the adventure of war. He had arrived in the first week to brief them on the Polish bombe, then flown back to Poland. When Poland fell he had fled to France, and when France collapsed he had escaped across the Pyrenees. Romantic stories clustered around him: that he had hidden from the Germans in a goatherd's cottage, that he had smuggled himself aboard a Portuguese steamer and ordered the captain to sail to England at pistol-point. When he had popped up again in Bletchley in the winter of 1940 it was Pinker, the Shakespearian, who had shortened his name to Puck ('that merry wanderer of the night'). His mother was British, which explained his almost perfect English, distinctive only because he pronounced it so carefully.

'You have come to give us assistance?'

'So it seems.' He shyly disengaged himself from Puck's embrace. 'For what it's worth.'

'Splendid, splendid.' Logie regarded them fondly for a moment, then began rummaging among the litter on his desk. 'Now where is that thing? It was here this morning'

Puck nodded at Logie's back and whispered: 'Do you see, Tom? As organised as ever.'

'Now, now, Puck, I heard that. Let me see. Is this it? No. Yes. Yes!'

He turned and handed Jericho a typewritten document, officially stamped and headed 'By Order of the War Office'. It was a billeting notice, served on a Mrs Ethel Armstrong, entitling Jericho to lodgings in the Commercial Guesthouse, Albion Street, Bletchley.

I'm afraid I don't know what it's like, old thing. Best I could do.'

'I'm sure it's fine.' Jericho folded the chit and stuffed it into his pocket. Actually, he was quite sure it warn't fine – the last decent rooms in Bletchley had disappeared three years ago, and people now had to travel in from as far away as Bedford, twenty miles distant – but what was the point in complaining? On past experience he wouldn't be using the room much anyway, except to sleep in.

'Now don't you go exhausting yourself, my boy,' said Logie. 'We don't expect you to work a full shift. Nothing like that. You just come and go as you please. What we want from you is what you gave us last time. Insight. Inspiration. Spotting that something we've all missed. Isn't that so, Puck?'

'Absolutely.' His handsome face was more haggard than Jericho had ever seen it, more tired even than Logie's. 'God knows, Tom, we are certainly up against it.'

'I take it then we're no further forward?' said Logie. 'No good news I can give our lord and master?'

Puck shook his head.

'Not even a glimmer?'

'Not even that.'

'No. Well, why should there be? Damn bloody admirals! Logie screwed up the message slip, aimed it at his rubbish bin and missed. 'I'd show you round myself, Tom, but the Skynner waits for no man, as you'll recall. All right with you, Puck? Give him the grand tour?'

'Of course, Guy. As you wish.'

Logie ushered them out into the passage and tried to lock the door, then gave up on it. As he turned he opened his mouth and Jericho nerved himself for one of Logie's excruciating housemaster's pep talks -something about innocent lives depending on them, and the need for them to do their best, and the race being not to the swift nor the battle to the strong (he had actually said this once) – but instead his mouth just widened into a yawn.

'Oh, dear. Sorry, old thing. Sorry.'

He shuffled off down the corridor, patting his pockets to make sure he had his pipe and tobacco pouch. They heard him mutter again, something about 'bloody admirals', and he was gone.

Hut 8 was thirty-five yards long by ten wide and Jericho could have toured it in his sleep, probably had toured it in his sleep, for all he knew. The outside walls were thin and the damp from the lake seemed to rise through the floorboards so that at night the rooms were chilly, cast in a sepia glow by bare, low-wattage bulbs. The furniture was mostly trestle tables and folding wooden chairs. It reminded Jericho of a church hall on a winter's night. All that was missing was a badly tuned piano and somebody thumping out 'Land of Hope and Glory'.

It was laid out like an assembly line, the main stage in a process that originated somewhere far out in the darkness, maybe two thousand miles away, when the grey hull of a U-boat rose close to the surface and squirted off a radio message to its controllers. The signals were intercepted at various listening-posts and teleprintered to Bletchley and within ten minutes of transmission, even as the U-boats were preparing to dive, they were emerging via a tunnel into Hut 8's Registration Room. Jericho helped himself to the contents of a wire basket labelled 'Shark' and carried them to the nearest light. The hours immediately after midnight were usually the busiest time. Sure enough, six messages had been intercepted in the last eighteen minutes. Three consisted of just eight letters: he guessed they were weather reports. Even the longest of the other cryptograms was no more than a couple of dozen four-letter groups:

JRLO GOPL DNRZ LOBT -

Puck made a weary face at him, as if to say: What can you do?

Jericho said: 'What's the volume?'

'It varies. One hundred and fifty, perhaps two hundred messages a day. And rising.'

The Registration Room didn't just handle Shark. There was Porpoise and Dolphin and all the other different Enigma keys to log and then pass across the corridor to the Crib Room. Here, the cribsters sifted them for clues – radio station call signs they recognised (Kiel was JDU, for example, Wilhelmshaven KYU), messages whose contents they could guess at, or cryptograms that had already been enciphered in one key and then retransmitted in another (they marked these 'XX' and called them 'kisses'). Atwood was the champion cribster and the Wrens said cattily behind his back that these were the only kisses he had ever had.

It was in the big room next door – which they called, with their solemn humour, the Big Room -that the cryptanalysts used the cribs to construct possible solutions that could be tested on the bombes. Jericho took in the rickety tables, the hard chairs, the weak lighting, the fug of tobacco, the college-library atmosphere, the night chill (most of the cryptanalysts were wearing coats and mittens) and he wondered why – why? – he had been so ready to come back. Kingcome and Proudfoot were there, and Upjohn and Pinker and de Brooke, and maybe half a dozen newcomers whose faces he didn't recognise, including one young man sitting bold as you please in the seat which had once been reserved for Jericho. The tables were stacked with cryptograms, like ballot papers at an election count.

Puck was muttering something about back-breaks but Jericho, fascinated by the sight of someone else in his place, lost track and had to interrupt him. 'I'm sorry, Puck. What was that?'

'I was saying that from twenty minutes ago we are up to date. Shark is now fully read to the point of the code change. So that there is nothing left to us. Except history.' He gave a weak smile and patted Jericho's shoulder. 'Come. I'll show you.'

When a cryptanalyst believed he'd glimpsed a possible break into a message, his guess was sent out of the hut to be tested on a bombe. And if he'd been skilful enough, or lucky enough, then in an hour, or a day, the bombe would churn through a million permutations and reveal how the Enigma machine had been set up. That information was relayed back from the bombe bays to the Decoding Room.

Because of its noise, the Decoding Room was tucked away at the far end of the hut. Personally, Jericho liked the clatter. It was the sound of success. His worst memories were of the nights when the building was silent. A dozen British Type-X enciphering machines had been modified to mimic the actions of the German Enigma. They were big, cumbersome devices -typewriters with rotors, a plugboard and a cylinder – at which sat young and well-groomed debutantes.

Baxter, who was the hut's resident Marxist, had a theory that Bletchley's workforce (which was mainly female) was arranged in what he called 'a paradigm of the English class system'. The wireless interceptors, shivering in their coastal radio stations, were generally working-class and laboured in ignorance of the Enigma secret. The bombe operators, who worked in the grounds of some nearby country houses and in a big new installation just outside London, were petit-bourgeois and had a vague idea. And the Decoding Room girls, in the heart of the Park, were mostly upper-middle-class, even aristocratic, and they saw it all – the secrets literally passed through their fingers. They typed out the letters of the original cryptogram, and from the cylinder on the right of the Type-X a strip of sticky-backed paper, the sort you saw gummed down on telegram forms, slowly emerged, bearing the decrypted plaintext.

'Those three are doing Dolphin,' said Puck, pointing across the room, 'and the two by the door are just starting on Porpoise. And this charming young lady here, I believe' – he bowed to her – 'has Shark. May we?'

She was young, about eighteen, with curly red hair and wide hazel eyes. She looked up and smiled at him, a dazzling Tatler smile, and he leaned across her and began uncoiling the strip of tape from the cylinder. Jericho noticed as he did so that he left one hand resting casually on her shoulder, just as simply as that, and he thought how much he envied Puck the ease of that gesture. It would have taken him a week to pluck up the nerve. Puck beckoned him down to read the decrypt.

VONSCHULZEQU8 8521DAMPFER1TANKERWAHRSCHEINLICHAM6 3TANKERFACKEL…

Jericho ran his finger along it, separating the words and translating it in his mind: U-boat commander von Schulze was in grid square 8852 and had sunk one steamship (for certain) and one tanker (probably) and had set one other tanker on fire…

'What date is this?'

'You can see it there,' said Puck. 'Sechs drei. The sixth of March. We've broken everything from this week up to the code change on Wednesday night, so now we go back and pick up the intercepts we missed earlier in the month. This is – what? – six days old. Herr Kapitan von Schulze may be five hundred miles away by now. It is of academic interest only, I fear.'

'Poor devils,' said Jericho, passing his finger along the tape for a second time. IDAMPFERITANKER… What freezing and drowning and burning were concentrated in that one line! What were the ships called, he wondered, and had the families of the crews been told?

'We have approximately a further eighty messages from the sixth still to run through the Type-Xs. I shall put two more operators on to it. A couple of hours and we should be finished.'

'And then what?'

'Then, my dear Tom? Then I suppose we shall make a start on back-breaks from February. But that barely qualifies even as history. February? February in the Atlantic? Archaeology!'

'Any progress on the four-wheel bombe?'

Puck shook his head. 'First, it is impossible. It is out of the question. Then there is a design, but the design is theoretical nonsense. Then there is a design that should work, but doesn't. Then there is a shortage of materials. Then there is a shortage of engineers…' He made a weary gesture with his hand, as if he were pushing it all out of the way.

'Has anything else changed?'

'Nothing that affects us. According to the direction finders, U-boat HQ has moved from Paris to Berlin. They have some wonderful new transmitter at Magdeburg they say will reach a U-boat forty-five feet under water at a range of two thousand miles.,'

Jericho murmured: 'How very ingenious of them.'

The red-headed girl had finished deciphering the message. She tore off the tape, stuck it on the back of the cryptogram and handed it to another girl, who rushed out of the room. Now it would be turned into recognisable English and teleprintered to the Admiralty.

Puck touched Jericho's arm. 'You must be tired. Why don't you go now and rest?'

But Jericho didn't feel like sleeping. 'I'd like to see all the Shark traffic we haven't been able to break. Everything since midnight on Wednesday.'

Puck gave a puzzled smile. 'Why? There's nothing you can do with it.'

'Maybe so. But I'd like to see it.'

'Why?'

'I don't know.' Jericho shrugged. 'Just to handle it. To get a feel of it. I've been out of the game for a month.'

'You think we may have missed something, perhaps?'

'Not at all. But Logie has asked me.'

'Ah yes. The celebrated Jericho "inspiration" and "intuition".' Puck couldn't conceal his irritation. 'And so from science and logic we descend to superstition and "feelings".'

'For heaven's sake, Puck!' Jericho was starting to become annoyed himself. 'Just humour me, if that's how you prefer to look at it.'

Puck glared at him for a moment, and then, as quickly as they had arisen, the clouds seemed to pass. 'Of course.' He held up his hands in a gesture of surrender. 'You must see it all. Forgive me. I'm tired. We're all tired.'

Five minutes later, when Jericho walked into the Big Room carrying the folder of Shark cryptograms, he found his old seat had been vacated. Someone had also laid out in his place a new pile of jotting paper and three freshly sharpened pencils. He looked around, but nobody seemed to be paying him any attention.

He laid the intercepts out on the table. He loosened his scarf. He felt the radiator – as ever, it was lukewarm. He blew some warmth on to his hands and sat down.

He was back.

3

Whenever anyone asked Jericho why he was a mathematician – some friend of his mother, perhaps, or an inquisitive colleague with no interest in science – he would shake his head and smile and claim he had no idea. If they persisted, he might, with some diffidence, direct them to the definition offered by G. H. Hardy in his famous Apology: 'a mathematician, like a painter or a poet, is a maker of patterns'. If that didn't satisfy them, he would try to explain by quoting the most basic illustration he could think of: pi – 3.14 – the ratio of a circle's circumference to its diameter. Calculate pi to a thousand decimal places, he would say, or a million or more, and you will discover no pattern to its unending sequence of digits. It appears random, chaotic, ugly. Yet Leibnitz and Gregory can take the same number and tease from it a pattern of crystalline elegance:

pi/4=1-1/3+1/5-1/7+1/9-

and so on to infinity. Such a pattern had no practical usefulness, it was merely beautiful – as sublime, to Jericho, as a line in a fugue by Bach – and if his questioner still couldn't see what he was driving at, then, sadly, he would give up on them as a waste of time.

On the same principle, Jericho thought the Enigma machine was beautiful – a masterpiece of human ingenuity that created both chaos and a tiny ribbon of meaning. In the early days at Bletchley he used to fantasise that some day, when the war was over, he would track down its German inventor, Herr Arthur Scherbius, and buy him a glass of beer. But then he'd heard that Scherbius had died in 1929, killed – of all ludicrously illogical things – by a runaway horse, and hadn't lived to see the success of his patent.

If he had, he would have been a rich man. By the end of 1942 Bletchley estimated that the German had manufactured at least a hundred thousand Enigmas. Every Army headquarters had one, every Luftwaffe base, every warship, every submarine, every port, every big railway station, every SS brigade and Gestapo HQ.

Never before had a nation entrusted so much of its secret communications to a single device.

In the mansion at Bletchley the cryptanalysts had a roomful of captured Enigmas and Jericho had played with them for hours. They were small (little more than a foot square by six inches deep), portable (they weighed just twenty-six pounds) and simple to operate. You set up your machine, typed in your message, and the ciphertext was spelled out, letter by letter, on a panel of small electric bulbs. Whoever received the enciphered message merely had to set up his machine in exactly the same way, type in the cryptogram, and there, spelled out on the bulbs, would be the original plaintext.

The genius lay in the vast number of different permutations the Enigma could generate. Electric current on a standard Enigma flowed from keyboard to lamps via a set of three wired rotors (at least one of which turned a notch every time a key was struck) and a plugboard with twenty-six jacks. The circuits changed constantly; their potential number was astronomical, but calculable. There were five different rotors to choose from (two were kept spare) which meant they could be arranged in any one of sixty possible orders. Each rotor was slotted on to a spindle and had twenty-six possible starting positions. Twenty-six to the power of three was 17,576. Multiply that by the sixty potential rotor-orders and you got 1,054,560. Multiply that by the possible number of plugboard connections – about 150 million million – and you were looking at a machine that had around 150 million million million different starting positions. It didn't matter how many Enigma machines you captured or how long you played with them. They were useless unless you knew the rotor order, the rotor starting positions and the plugboard connections. And the Germans changed these daily, sometimes twice a day.

The machine had only one tiny – but, as it turned out, crucial – flaw. It could never encipher a letter as itself: an A would never emerge from it as an A, or a B as a B, or a C as a C… Nothing is ever itself: that was the great guiding principle in the breaking of Enigma, the infinitesimal weakness that the bombes exploited.

Suppose one had a cryptogram that began:

IGWH BSTU XNTX EYLK PEAZ ZNSK UFJR CADV _

And suppose one knew that this message originated from the Kriegsmarine's weather station in the Bay of Biscay, a particular friend of the Hut 8 cribsters, which always began its reports in the same way:

WEUBYYNULLSEQSNULLNULL

('Weather survey 0600', WEUB being an abbreviation for WETTERUBERSICHT and SEQS for SECHS; YY and NULL being inserted to baffle eavesdroppers).

The cryptanalyst would lay out the ciphertext and slide the crib beneath it and on the principle that nothing is ever itself he would keep sliding it until he found a position in which there were no matching letters between the top and bottom lines. The result in this case would be:

BSTUXNTXEYLKPEAZZNSKUF

WEUBYYNULLSEQSNULLNULL

And at this point it became theoretically possible to calculate the original Enigma settings that alone could have produced this precise sequence of letter pairings. It was still an immense calculation, one which would have taken a team of human beings several weeks. The Germans assumed, rightly, that whatever intelligence might be gained would be too old to be of use. But Bletchley – and this was what the Germans had never reckoned on – Bletchley didn't use human beings. It used bombes. For the first time in history, a cipher mass-manufactured by machine was being broken by machine.

Who needed spies now? What need now of secret inks and dead-letter drops and midnight assignations in curtained wagons-lits? Now you needed mathematicians and engineers with oilcans and fifteen hundred filing clerks to process five thousand secret messages a day. They had taken espionage into the machine age.

But none of this was of much help to Jericho in breaking Shark.

Shark defied every tool he could bring to bear on it. For a start, there were almost no cribs. In the case of a surface Enigma key, if Hut 8 ran out of cribs, they had tricks to get round it – 'gardening', for example. 'Gardening' was arranging for the RAF to lay mines in a particular naval grid square outside a German harbour. An hour later, you could guarantee, the harbour master, with Teutonic efficiency, would send a message using that day's Enigma settings, warning ships to beware of mines in naval grid square such-and-such. The signal would be intercepted, flashed to Hut 8, and give them their missing crib.

But you couldn't do that with Shark and Jericho could make only the vaguest guesses at the contents of the cryptograms. There were eight long messages originating from Berlin. They would be orders, he supposed, probably directing the U-boats into 'wolf packs' and stationing them in front of the oncoming convoys. The shorter signals – there were a hundred and twenty-two, which Jericho sorted into a separate pile – had been sent by the submarines themselves. These could contain anything: reports of ships sunk and of engine trouble; details of survivors floating in the water and of crewmen washed overboard; requests for spare parts and fresh orders. Shortest of all were the U-boats' weather messages or, very occasionally, contact reports: 'Convoy in naval grid square BE9533 course 70 degrees speed 9 knots…' But these were encoded, like the weather bulletins, with one letter of the alphabet substituting for each piece of information. And then they were enciphered in Shark.

He tapped his pencil against the desk. Puck was quite right. There was not enough material to work with.

And even if there had been, there was still the wretched fourth rotor on the Shark Enigma, the innovation that made U-boat messages twenty-six times more difficult to break than those of surface ships. One hundred and fifty million million million multiplied by twenty-six. A phenomenal number. The engineers had been struggling for a year to develop a four-rotor bombe – but still, apparently, without success. It seemed to be just that one step beyond their technical ability.

No cribs, no bombes. Hopeless.

Hours passed during which Jericho tried every trick he could think of to prompt some fresh inspiration. He arranged the cryptograms chronologically. Then he arranged them by length. Then he sorted them by frequency. He doodled on the pile of paper. He prowled around the hut, oblivious now to who was looking at him and who wasn't. This was what it had been like for ten interminable months last year. No wonder he had gone mad. The chorus-lines of meaningless letters danced before his eyes. But they were not meaningless. They were loaded with the most vital meaning imaginable, if only he could find it. But where was the pattern? Where was the pattern? Where was the pattern?

It was the practice on the night shift at about four o'clock in the morning for everyone to take a meal-break. The cryptanalysts went off when they liked, depending on the stage they'd reached in their work. The Decoding Room girls and the clerks in the Registration and Catalogue rooms had to leave according to a rota so that the hut was never caught short-staffed.

Jericho didn't notice the drift of people towards the door. He had both elbows on the table and was leaning over the cryptograms, his knuckles pressed to his temples. His mind was eidetic – that is to say, it could hold and retrieve images with photographic accuracy, be they mid-game positions in chess, crossword puzzles or enciphered German naval signals – and he was working with his eyes closed.

' "Below the thunders of the upper deep,"' intoned a muffled voice behind him, '"Far, far beneath in the abysmal sea,/His ancient, dreamless, uninvaded sleep…"'

'"… The Kraken sleepeth.'" Jericho finished the quotation and turned to find Atwood pulling on a purple balaclava. 'Coleridge?'

'Coleridge?' Atwood's face abruptly emerged wearing an expression of outrage. 'Coleridge? It's Tennyson, you barbarian. We wondered whether you'd care to join us for refreshment.'

Jericho was about to refuse, but decided that would be rude. In any case, he was hungry. He'd eaten nothing except toast and jam for twelve hours.

'That's kind. Thank you.'

He followed Atwood, Pinker and a couple of the others along the length of the hut and out into the night. At some stage while he'd been lost in the cryptograms it must have rained and the air was still moist. Along the road to the right he could hear people moving in the shadows. The beams of torches glistened on the wet tarmac. Atwood conducted them past the mansion and the arboretum and through the main gate. Discussing work outside the hut was forbidden and Atwood, purely to annoy Pinker, was declaiming on the suicide of Virginia Woolf, which he held to be the greatest day for English letters since the invention of the printing press.

'I c-c-can't believe you mmm-mmm-mmm…' When Pinker snagged himself on a word, his whole body seemed to shake with the effort of trying to get himself free. Above his bow-tie, his face bloomed scarlet in the torchlight. They stopped and waited patiently for him. 'Mmm-mmm…'

'Mean that?' suggested Atwood.

'Mean that, Frank,' gasped Pinker with relief. 'Thank you.'

Someone came to Atwood's support, and then Pinker's shrill voice started to argue again. They moved off. Jericho lagged behind.

The canteen, which lay just behind the perimeter fence, was as big as an aircraft hangar, brightly lit and thunderously noisy, with perhaps five or six hundred people sitting down to eat or queuing for food.

One of the new cryptanalysts shouted to Jericho: 'I bet you've missed this!' Jericho smiled and was about to say something in return but the young man went off to collect a tray. The din was dreadful, and so was the smell – a blended steam of institutional food, of cabbage and boiled fish and custard, laced with cigarette smoke and damp clothes. Jericho felt simultaneously intimidated by it and detached from it, like a prisoner returning from solitary confinement, or a patient from an isolation ward released on to the street after a long illness.

He queued and didn't pay much attention to the food being slopped on his plate. It was only after he had handed over his two shillings and sat down that he took a good look at it – boiled potatoes in a curdled yellow grease and a slab of something ribbed and grey. He stabbed at the lump with his fork, then lifted a fragment cautiously to his mouth. It tasted like fishy liver, like congealed cod liver oil. He winced.

'This is perfectly vile.'

Atwood said, through a full mouth: 'It's whale meat.'

'Good heavens.' Jericho put his fork down hurriedly.

'Don't waste it, dear boy. Don't you know there's a war on? Pass it over.'

Jericho pushed the plate across the table and tried to swill the taste away with the milk-water coffee.

The pudding was some kind of fruit tart, and that was better, or, rather, it tasted of nothing more noxious than cardboard, but halfway through it, Jericho's wavering appetite finally died. Atwood was now giving them his opinion of Gielgud's interpretation of Hamlet, spraying the table in the process with particles of whale, and at that point Jericho decided he'd had enough. He took the leftovers that Atwood didn't want and scraped them into a milk churn labelled 'PIG SWILL'.

When he was halfway to the door he was suddenly overcome with remorse at his rudeness. Was this the behaviour of a good colleague, what Skynner would call 'a team player'? But then, when he turned and looked back, he saw that nobody had missed him. Atwood was still talking, waving his fork in mid-air, Pinker was shaking his head, the others were listening.

Jericho turned once more for the door and the salvation of the fresh air.

Thirty seconds later he was out on the pavement, picking his way carefully in the darkness towards the guard post, thinking about Shark.

He could hear the click click of a woman's heels hurrying about twenty paces in front of him. There was no one else around. It was between sittings: everyone was either working or eating. The rapid footsteps stopped at the barrier and a moment later the sentry shone his torch directly in the woman's face. She glanced away with a murmur of annoyance, and Jericho saw her then, for an instant, spot-lit in the blackout, looking straight in his direction.

It was Claire.

For a fraction of a second, he thought she must have seen him. But he was in the shadows and reeling backwards in panic, four or five steps backwards, and she was dazzled by the light. With what seemed like infinite slowness she brought her hand up to shield her eyes. Her blonde hair gleamed white.

He couldn't hear what was said but very quickly the torch was quenched and everything was dark again. And then he heard her moving off down the path on the other side of the barrier, click click click, obviously in a rush about something, fading into the night.

He had to catch her up. He stumbled quickly to the guard post, searching for his wallet, searching for his pass, nearly tripping off the kerbstone, but he couldn't find the damned thing. The torch came on, blinding him – 'evening sir', 'evening corporal'- and his fingers were useless, he couldn't make them work, and the pass wasn't in his wallet, wasn't in his overcoat pockets, wasn't in his jacket pockets, breast pocket – he couldn't hear her footsteps now, just the sentry's boot tapping impatiently – and, yes, it was in his breast pocket, 'here you are', 'thank you sir', 'thank you corporal', 'night sir', 'night corporal', night, night, night…

She was gone.

The sentry's light had robbed him of what little vision he had. When he closed his eyes there was only the imprint of the torch and when he opened them the darkness was absolute. He found the edge of the road with his foot and followed its curve. It took him once again past the mansion and brought him out close to the huts. Far away, on the opposite bank of the lake, someone – perhaps another sentry – started to whistle 'We'll Gather Lilacs in the Spring Again', then stopped.

It was so quiet, he could hear the wind moving in the trees.

While he was hesitating, wondering what to do, a dot of light appeared along the footpath to his right, and then another. For some reason Jericho drew back into the shadows of Hut 8 as the torches bobbed towards him. He heard voices he didn't recognise – a man's and a woman's – whispered but emphatic. When they were almost level with him, the man threw his cigarette into the water. A cascade of red points ended in a hiss. The woman said: 'It's just a week, darling,' and went to embrace him. The fireflies danced and separated and moved on.

He stepped out onto the path again. His night vision was coming back. He looked at his watch. It was 4.30. Another ninety minutes and it would start to get light.

On impulse he walked down the side of Hut 8, keeping close to the blastproof wall. This brought him to the edge of Hut 6, where the ciphers of the German Army and Luftwaffe were broken. Straight ahead was a narrow alleyway of rough grass separating Hut 6 from the end wall of the Naval Section. And at the end of that, crouched low in the darkness, just about visible, was the side of another hut – Hut 3 – to which the decrypted ciphers from Hut 6 were sent for translation and dispatch.

Hut 3 was where Claire worked.

He glanced around. There was no one in sight.

He left the path and started to stumble down the passage. The ground was slippery and uneven and several times something grabbed at his ankle – ivy, maybe, or a tendril of discarded cable – and almost sent him sprawling. It took him about a minute to reach Hut 3.

Here, too, was a concrete wall, designed, optimistically, to shield the flimsy wooden structure from an exploding bomb. It was neck-high, but although he was short he was just about able to peer over the top.

A row of windows was set into the side of the building. Over these, from the outside, blackout shutters were fastened every day at dusk. All that was visible was the ghosts of squares, where the light seeped around the edges of the frames. The floor of Hut 3, like Hut 8's, was made of wood, suspended above a concrete base, and he could hear the muffled clumps and thuds of people moving about.

She must be on duty. She must be working the midnight shift. She might be three feet from where he stood.

He was on tiptoe.

He had never been inside Hut 3. For reasons of security, workers in one section of the Park were not encouraged to stray into another, not unless they had good reason. From time to time his work had taken him over the threshold of Hut 6, but Hut 3 was a mystery to him. He had no idea of what she did. She'd tried to tell him once, but he'd said gently that it was best he didn't know. From odd remarks he gathered it was something to do with filing and was 'deadly dull, darling'.

He stretched out as far as he could, until his fingertips were brushing the asbestos cladding of the hut.

What are you doing, darling Claire? Are you busy with your boring filing, or are you flirting with one of the night-duty officers, or gossiping with the other girls, or puzzling over that crossword you can never do?

Suddenly, about fifteen yards to his left, a door opened. From the oblong of dim light a uniformed man emerged, yawning. Jericho slid silently to the ground until he was kneeling in the wet earth and pressed his chest against the wall. The door closed and the man began to walk towards him. He stopped about ten feet away, breathing hard. He seemed to be listening. Jericho closed his eyes and shortly afterwards he heard a pattering and then a drilling noise and when he opened them he saw the faint silhouette of the man pissing against the wall, very hard. It went on for a wondrously long time and Jericho was close enough to get a whiff of pungent, beery urine. A fine spray was being borne downwind on the breeze. He had to put his hand to his nose and mouth to stop himself gagging. Eventually, the man gave a deep sigh – a groan, almost – of satisfaction, and fumbled with the buttons of his fly. He moved away. The door opened and closed again and Jericho was alone.

There was a certain humour in the situation, and later even he was to see it. But at the time he was on the edge of panic. What, in the name of reason, did he think he was doing? If he were to be caught, kneeling in the darkness, with his ear pressed to a hut in which he had no business, he would have – to put it mildly -a hard time explaining himself. For a moment he considered simply marching inside and demanding to see her. But his imagination recoiled at the prospect. He might be thrown out. Or she might appear in fury and create a scene. Or she might appear and be the soul of sweetness, in which case what did he say? 'Oh, hello, darling. I just happened to be passing. You look in good form. By the way, I've been meaning to ask you, why did you wreck my life?'

He used the wall to help him scramble to his feet. The quickest way back to the road was straight head, but that would take him past the door of the hut. He decided that the safest course would be to go back the way he had come.

He was more cautious after his scare. Each time he took a step he planted his foot carefully and on every fifth pace he paused to make sure that no one else was moving around in the blackout. Two minutes later he was back outside the entrance to Hut 8.

He felt as if he had been on a cross-country run. He was out of breath. There was a small hole in his left shoe and his sock was wet. Bits of damp grass were sticking to the bottoms of his trouser legs. His knees were sodden. And where he had rubbed against the concrete wall the front of his overcoat was streaked luminously white. He took out his handkerchief and tried to clean himself up.

He had just about finished when he heard the others coming back from the canteen. Atwood's voice carried in the night: 'A dark horse, that one. Very dark. I recruited him, you know,' to which someone else chimed in: 'Yes, but he was once very good, wasn't he?'

Jericho didn't stop to hear the rest. He pushed open the door and almost ran down the passage, so that by the time the cryptanaiysts appeared in the Big Room he was already seated at his desk, bent over the intercepts, knuckles to his temples, eyes closed.

He stayed like that for three hours.

At about six o'clock, Puck stopped by to drop on the table another forty encrypted signals, the latest batch of Shark traffic, and to enquire – not without a degree of sarcasm – if Jericho had 'solved it yet?' At seven, there was a rattle of step ladders against the outside wall and the blackout shutters were unfastened. A pale grey light filtered into the hut.

What was she doing, hurrying into the Park at that time of night? That was what he did not understand. Of course, the mere fact of seeing her again after a month spent tiying to forget her was disturbing. But it was the circumstances, in retrospect, that troubled him more. She had not been in the canteen, he was sure of that. He had scrutinised every table, every face – had been so distracted he had barely even looked at what he was being given to eat. But if she had not been in the canteen, where had she been? Had she been with someone? Who? Who? And the way she was walking so hurriedly. Was there not something furtive, even panicky, about it?

His memory replayed the scene frame by frame: the footsteps, the flash of light, the turn of her head, her cry, the halo of her hair, the way she had vanished… That was something else. Could she really have walked the entire distance to the hut in the time it took him to fumble for his pass?

Just before eight o'clock he gathered the cryptograms together and slipped them into the folder. All around him, the cryptanaiysts were preparing to go off shift – stretching and yawning and rubbing at tired eyes, pulling together their work, briefing their replacements. Nobody noticed Jericho walk quickly down the corridor to Logie's office. He knocked once. There was no reply. He tried the door. As he remembered: unlocked.

He closed it behind him and picked up the telephone. If he delayed for a second his nerve would fail him. He dialled '0' and on the seventh ring, just as he was about to give up, a sleepy operator answered.

His mouth was almost too dry to get the words out. 'Duty Officer, Hut 3, please.'

Almost immediately a man's voice said, irritably: 'Colonel Coker.'

Jericho nearly dropped the receiver.

'Do you have a Miss Romilly there?' He didn't need to disguise his voice: it was so strained and quavering it was unrecognisable. 'A Miss Claire Romilly?'

'You've come through to completely the wrong office. Who is this?'

'Welfare.'

'Oh bloody hell! There was a deafening bang, as if the colonel had thrown the telephone across the room, but the connection held. Jericho could hear the clatter of a teleprinter and a man's voice, very cultured, somewhere in the background: 'Yes, yes, I've got that. Right-oh. Cheerio.' The man ended one conversation and started another. 'Army Index here…' Jericho glanced at the clock above the window. Now it was past eight. Come on, come on… Suddenly there was more loud banging, much closer, and a woman said softly in Jericho's ear: 'Yes?'

He tried to sound casual but it came out as a croak. 'Claire?'

'No, I'm afraid it's Claire's day off. She won't be back on duty until eight tomorrow morning. Can I help?'

Jericho gently replaced the receiver in its cradle, just as the door was thrown open behind him.

'Oh, there you are, old thing…'

4

Daylight diminished the huts.

The blackout had touched them with a certain mystery but the morning showed them up for what they were: squat and ugly, with brown walls and tarred roofs and a premature air of dereliction. Above the mansion, the sky was glossy white with streaks of grey, a dome of polished marble. A duck in drab winter plumage waddled across the path from the lake looking for food, and Logie almost kicked it as he strode past, sending it protesting back to the water.

He had not been in the least perturbed to find Jericho in his office and Jericho's carefully prepared excuse – that he was returning the Shark intercepts had been waved away.

'Just dump 'em in the Crib Room and come with me.'

Drawn across the northern edge of the lake, next to the huts, was A-Block, a long, two-storey affair with brick walls and a flat top. Logie led the way up a flight of concrete steps and turned right. At the far end of the corridor a door opened and Jericho heard a familiar voice booming: '… all our resources, human and material, into this problem…' and then the door closed again and Baxter peered down the passage towards them.

'So there you are. I was just coming to find you. Hello, Guy. Hello, Tom. How are you? Hardly recognised you. Upright.' Baxter had a cigarette in his mouth and didn't bother to remove it, so it bobbed as he spoke and sprayed ash down the front of his pullover. Before the war he had been a lecturer at the London School of Economics.

'What have we got?' said Logie, nodding towards the closed door.

'Our American "lee-ay-son officer", plus another American – some big shot from the Navy. A man in a suit – a lounge lizard from Intelligence by the look of him. Three from our Navy, of course, one of them an admiral. All up from London specially.'

'An admiral?' Logie's hand went automatically to his tie and Jericho noticed he had changed into a pre-war double-breasted suit. He licked his fingers and tried to plaster down his hair. 'I don't like the sound of an admiral. And how's Skynner?'

'At the moment? I'd say heavily out-gunned.' Baxter was staring at Jericho. The corners of his mouth twitched down briefly, the nearest Jericho had seen him come to a smile. 'Well, well, I suppose you don't look too bad, Tom.'

'Now, Alec, don't you go upsetting him.'

I'm fine, Alec, thank you. How's the revolution?'

'Coming along, comrade. Coming along.'

Logie patted Jericho on the arm. 'Don't say anything when we get inside, Tom. You're only here for show, old love.'

Only here for show, thought Jericho, what the hell does that mean? But before he could ask, Logie had opened the door and all he could hear was Skynner -'we must expect these setbacks from time to time' -and they were on.

There were eight men in the room. Leonard Skynner, the head of the Naval Section, sat at one end of the table, with Atwood to his right and an empty chair to his left, which Baxter promptly reclaimed. Gathered around the other end were five officers in dark blue naval uniform, two of them American and three British. One of the British officers, a lieutenant, had an eye-patch. They looked grim.

The eighth man had his back to Jericho. He turned as they came in and Jericho briefly registered a lean face with fair hair.

Skynner stopped speaking. He stood and held out a meaty hand. 'Come in Guy, come in Tom.' He was a big square-faced man with thick black hair and wide bushy eyebrows that almost met above the bridge of his nose and reminded Jericho of the Morse code symbol for M. He beckoned to the newcomers eagerly, obviously thankful to see Allied reinforcements. 'This is Guy Logie,' he said to the admiral, 'our chief cryptanalyst, and Tom Jericho, of whom you may have heard. Tom was instrumental in getting us into Shark just before Christmas.'

The admiral's leathery old face was immobile. He was smoking a cigarette – they were all smoking cigarettes except for Skynner- and he regarded Jericho, as did the Americans, blankly, through a fog of tobacco, without the slightest interest. Skynner rattled off the introductions, his arm sweeping round the table like the hand of a clock. 'This is Admiral Trowbridge. Lieutenant Cave. Lieutenant Villiers. Commander Hammerbeck -' the older of the two Americans nodded '- Lieutenant Kramer, US Navy Liaison. Mr Wigram is observing for the cabinet Office.' Skynner gave a little bow to everybody and sat down again. He was sweating.

Jericho and Logie each collected a folding chair from a stack beside the table and took up positions next to Baxter.

Almost the whole of the wall behind the admiral was taken up by a map of the North Atlantic. Clusters of coloured discs showed the positions of Allied convoys and their escorts: yellow for the merchantmen, green for the warships. Black triangles marked the suspected whereabouts of German U-boats. Beneath the chart was a red telephone, a direct link to the Submarine Tracking Room in the basement of the Admiralty. The only other decoration on the whitewashed walls was a pair of framed photographs. One was of the King, signed, looking nervous, presented after a recent visit. The other was of Grand Admiral Karl Donitz, commander in chief of the German Navy: Skynner liked to think of himself as locked in a personal battle with the wily Hun.

Now, though, he seemed to have lost the thread of what he was saying. He sorted through his notes and in the time it took Logie and Jericho to take their places, one of the Royal Navy men – Cave, the one with the eye-patch – received a nod from the admiral and started speaking.

'Perhaps, if you've finished outlining your problems, it might be helpful for us now to set out the operational situation.' His chair scraped on the bare floor as he rose to his feet. His tone was insultingly polite. 'The position at twenty-one hundred'

Jericho passed his hand over his unshaven chin. He couldn't make up his mind whether to keep his overcoat on or take it off. On, he decided – the room was cold, despite the number of people in it. He undid the buttons and loosened his scarf. As he did so, he noticed the admiral watching him. They couldn't believe it, these senior officers, whenever they came up to visit – the lack of discipline, the scarves and cardigans, the first-name terms. There was a story about Churchill, who'd visited the Park in 1941 and given a speech to the cryptanalysts on the lawn. Afterwards, as he was being driven away, he'd said to the director: 'When I told you to leave no stone unturned recruiting for this place, I didn't expect you to take me literally.' Jericho smiled at the memory. The admiral glowered and flicked cigarette ash on the floor.

The one-eyed naval officer had picked up a pointer and was standing in front of the Atlantic chart, holding a sheaf of notes.

'It must be said, unfortunately, that the news you've given us couldn't have come at a worse moment. No fewer than three convoys have left the United States in the past week and are presently at sea. Convoy SC-122.' He rapped it once with the pointer, hard, as if he had a grudge against it, and read out his notes. 'Departed New York last Friday. Carrying fuel oil, iron ore, steel, wheat, bauxite, sugar, refrigerated meat, zinc, tobacco and tanks. Fifty merchant ships.'

Cave spoke in a clipped, metallic voice, without looking at his audience. His one good eye was fixed on the map.

'Convoy HX-229.' He tapped it. 'Departed New York Monday. Forty merchant vessels. Carrying meat, explosives, lubricating oil, refrigerated dairy produce, manganese, lead, timber, phosphate, diesel oil, aviation spirit, sugar and powdered milk.' He turned to them for the first time. The whole of the left side of his face was a mass of purple scar tissue. 'That, I might say, is two weeks' supply of powdered milk for the entire British Isles.'

There was some nervous laughter. 'Better not lose that,' joked Skynner. The laughter stopped at once. He looked so forlorn in the silence Jericho almost felt sorry for him.

Again, the pointer crashed down.

'And Convoy HX-229A. Left New York Tuesday. Twenty-seven ships. Similar cargoes to the others. Fuel oil, aviation spirit, timber, steel, naval diesel, meat, sugar, wheat, explosives. Three convoys. A total of one hundred and seventeen merchant ships, with a gross registered tonnage of just under one million tons, plus cargo of another million.'

One of the Americans – it was the senior one, Hammerbeck – raised his hand. 'How many men involved?'

'Nine thousand merchant seamen. One thousand passengers.'

'Who are the passengers?'

'Mainly servicemen. Some ladies from the American Red Cross. Quite a lot of children. A party of Catholic missionaries, curiously enough.'

'Jesus Christ.'

Cave permitted himself a crimped smile. 'Quite.'

'And whereabouts are the U-boats?'

'Perhaps I might let my colleague answer that.'

Cave sat down and the other British officer, Villiers, took the floor. He flourished the pointer.

'Submarine Tracking Room had three U-boat packs operational as of zero-zero-hundred Thursday – heah, heah, and heah.' His accent barely qualified as recognisable English, it was the sort that pronounced 'cloth' as 'clawth' and 'really' as 'rarely', and when he spoke his lips hardly moved, as if it were somehow ungentlemanly – a betrayal of the amateur ethos – to put too much effort into talking. 'Gruppe Raubgraf heah, two hundred miles off the coast of Greenland. Gruppe Neuland, heah, almost precisely mid-ocean. And Gruppe Westmark heah, due south of Iceland.'

'Zero-zero Thursday! You mean more than thirty hours ago?' Hammerbeck's hair was the colour and thickness of steel wool, close-cropped to his scalp. It glinted in the fluorescent light as he leaned forwards. 'Where the hell are they now?'

I'm afraid I've no idea. I thought that was why we were heah. They've blipped awf the screen.'

Admiral Trowbridge lit another cigarette from the tip of his old one. He had transferred his attention from Jericho and now he was staring at Hammerbeck through small, rheumy eyes.

Again, the American raised his hand. 'How many subs are we talking about in these three wolf packs?'

I'm sorry to say, ah, they're quite large, ah, we estimate forty-six.'

Skynner squirmed in his chair. Atwood made a great show of rummaging through his papers.

'Let me get this straight,' said Hammerbeck. (He was certainly persistent – Jericho was beginning to admire him.) 'You're telling us one million tons of shipping -'

'Merchant shipping,' interrupted Cave. '

'- merchant shipping, pardon me, one million tons of merchant shipping, with ten thousand people on board, including various ladies of the American Red Cross and assorted Catholic Bible-bashers, is steaming towards forty-six U-boats, and you have no idea where those U-boats are?'

'I'm rather afraid I am, yes.'

'Well, I'll be fucked,' said Hammerbeck, sitting back in his chair. 'And how long before they get there?' 'That's hard to say.' It was Cave again. He had an odd habit of turning his face away when he talked, and Jericho realised he was trying not to show his shattered cheekbone. 'The SC is the slower convoy. She's making about seven knots. The HXs are both faster, one ten knots, one eleven. I'd say we've got three days, at the maximum. After that, they'll be within operational range of the enemy.'

Hammerbeck had begun whispering to the other American. He was shaking his head and making short chopping motions with his hand. The admiral leaned over and muttered something to Cave, who said quietly: 'I'm afraid so, sir.'

Jericho looked up at the Atlantic, at the yellow discs of the convoys and the black triangles of the U-boats, sewn like shark's teeth across the sea lanes. The distance between the ships and the wolf packs was roughly eight hundred miles. The merchantmen were making maybe two hundred and forty miles every twenty-four hours. Three days was about right. My God, he thought, no wonder Logie was so desperate to get me back.

'Gentlemen, please, if I may?' said Skynner loudly, bringing the meeting back to order. Jericho saw he'd plastered on his 'come let us smile in the face of disaster' expression – invariably a sign of incipient panic. 'I think we should guard against too much pessimism. The Atlantic does cover thirty-two million square miles, you know.' He risked another laugh. 'That's an awful lot of ocean.'

'Yes,' said Hammerbeck, 'and forty-six is one hell of a lot of U-boats.'

'I agree. It's probably the largest concentration of hearses we've faced,' said Cave. 'I'm afraid we must assume the enemy will make contact. Unless, of course, we can find out where they are.'

He gave Skynner a significant look, but Skynner ignored it and pressed on. 'And let's not forget – these convoys are not unprotected?' He glanced around the table for support. 'They do have an escort?'

'Indeed.' Cave again, 'They have an escort of- ' he consulted his notes '- seven destroyers, nine corvettes and three frigates. Plus various other vessels.'

'Under an experienced commander…'

The British officers glanced at one another, and then at the admiral.

'Actually, it's his first command.'

'Jesus Christ!' Hammerbeck rocked forwards in his chair and brought his fists down on the table.

'If I might step in heah. Obviously, we didn't know last Friday when the escorts were forming up that our intelligence was going to be shut awf.'

'How long will this blackout last?' This was the first time the admiral had spoken and everyone turned to look at him. He gave a sharp, explosive cough, which sounded as if small pieces of machinery were flying around loose in his chest, then sucked in another lungful of smoke and gestured with his cigarette. 'Will It be over in four days, d'you think?'

The question was addressed directly to Skynner and they all turned to look at him. He was an administrator, not a cryptanalyst – he'd been vice-chancellor of some northern university before the war – and Jericho knew he hadn't a clue. He didn't know whether the blackout would last four days, four months or four years.

Skynner said carefully: 'It's possible.'

'Yes, well, all things are possible.' Trowbridge gave an unpleasanr rasping laugh that turned into another cough. 'Is it likely? Is it likely you can break this, whatever you call it – this Shark – before our convoys come within range of the U-boats?'

'We'll give it every priority.'

'I know damn well you'll give it every priority, Leonard. You keep saying you'll give it every priority. That's not the question.'

'Well, sir, as you press me, sir, yes.' Skynner stuck his big jaw out heroically. In his mind's eye he was sreering his ship manfully into the face of the typhoon. 'Yes, I think we may be able to do it.'

You're mad, thought Jericho.

'And you all believe that?' The admiral stared hard in their direction. He had eyes like a bloodhound's, red-lidded and watery.

Logie was the first to break the silence. He looked at Skynner and winced and scratched the back of his head with the stem of his pipe. 'I suppose we do have the advantage of knowing more about Shark than we did before.'

Atwood jumped in: 'If Guy thinks we can do it, I certainly respect his opinion. I'd go along with whatever he estimates.' Baxter nodded judiciously. Jericho inspected his watch.

'And you?' said the admiral. 'What do you think?'

In Cambridge, they would just about be finishing breakfast. Kite would be steaming open the mail. Mrs Sax would be rattling round with her brushes and pails. In Hall on Saturday they served vegetable pie with potatoes for lunch…

He was aware that the room had gone quiet and he looked up to find all eyes were on him. The fair-haired man in the suit was slating at him with particular curiosity. He felt his face begin to colour.

And then he felt a spasm of irritation.

Afterwards Jericho was to think about this moment many times. What made him act as he did? Was it tiredness? Was he simply disoriented, plucked out of Cambridge and set down in the middle of this nightmare? Was he still ill? Illness would certainly help explain what happened later. Or was he so distracted by the thought of Claire that he wasn't thinking straight? All he remembered for certain was an overwhelming feeling of annoyance. 'You 're only here for show, old love.' You're only here to make up the numbers, so Skynner can put on a good act for the Yanks. You're only here to do as you're told, so keep your views to yourself, and don't ask questions. He was suddenly sick of it all, sick of everything – sick ot the blackout, sick of the cold, sick of the chummy first-name terms and the lime smell and the damp and the whale meat -whale meat- at four o'clock in the morning…

'Actually, I'm not sure I am as optimistic as my colleagues.'

Skynner interrupted him at once. You could almost hear the klaxons going off in his mind, see the airmen sprinting across the deck and the big guns swivelling skywards as HMS Skynner came under threat. 'Tom's been ill, sir, I'm afraid. He's been away from us for the best part of a month

'Why not?' The admiral's tone was dangerously friendly. 'Why aren't you optimistic?'

'… so I'm not sure he's altogether fully aut fait with the situation. Wouldn't you admit that, Tom?'

'Well, I'm certainly au fait with Enigma, ah, Leonard.' Jericho could hardly believe his own words. He plunged on. 'Enigma is a very sophisticated cipher system. And Shark is its ultimate refinement. I've spent the past eight hours reviewing the Shark material and, ah, forgive me if I'm speaking out of turn, but it seems to me we are in a very serious situation.'

'But you were breaking it successfully?'

'Yes, but we'd been given a key. The weather code was the key that unlocked the door. The Germans have now changed the weather code. That means we've lost our key. Unless there's been some development I'm not aware of, I don't understand how we're going to…' Jericho searched for a metaphor. '… pick the lock.'

The other American naval officer, the one who hadn't spoken so far – Jericho had momentarily forgotten his name – said: 'And you still haven't gotten those four-wheel bombes you promised us, Frank.'

'That's a separate issue,' muttered Skynner. He gave Jericho a murderous look.

'Is it?' Kramer – that was it. He was called Kramer. 'Surely if we had a few four-wheel bombes right now we wouldn't need the weather cribs?'

'Just stop there for a moment,' said the admiral, who had been following this conversation with increasing impatience. 'I'm a sailor, and an old sailor at that. I don't understand all this – talk- about keys and cribs and bombs with wheels. We're trying to keep the sea-lanes open from America and if we can't do that we're going to lose this war.'

'Hear, hear,' said Hammerbeck. 'Well said, Jack.'

'Now will somebody please give me a straight answer to a straight question? Will this blackout definitely be over in four days' time or won't it? Yes or no?

Skynner's shoulders sagged. 'No,' he said wearily. 'If you put it like that, sir, I can't say definitely it will be over, no.'

'Thank you. So, if it isn't over in four days, when will it be over? You. You're the pessimist. What do you think?'

Once again Jericho was conscious of everyone watching him.

He spoke carefully. Poor Logie was peering inside his tobacco pouch as if he wished he could climb in and never come out 'It's very hard to say. All we have to measure it by is the last blackout.'

'And how long did that go on?'

'Ten months.'

It was as if he had detonated a bomb. Everybody made a noise. The Navy men shouted. The admiral started coughing. Baxter and Atwood said 'No!' simultaneously. Logie groaned. Skynner, shaking his head, said: 'That really is defeatist of you, Tom.' Even Wigram, the fair-headed man, gave a snort and stared at the rafters, smiling at some private joke.

'I'm not saying it will definitely take us ten months,' Jericho resumed when he could make himself heard. 'But that's the measure of what we're up against and I think that four days is unrealistic. I'm sorry. I do.'

There was a pause, and then Wigram said, softly: ' Why, I wonder…'

'Mr Wigram?'

'Sorry, Leonard.' Wigram bestowed his smile around the table, and Jericho's immediate thought was how expensive he looked – blue suit, silk tie, Jermyn Street shirt, pomaded hair swept back and scented with some masculine cologne – he might have stepped out of the lobby of the Ritz. A lounge lizard, Baxter had called him, which was Bletchley code for spy.

'Sorry,' Wigram said again. 'Thinking aloud. I was just wondering why Donitz should have decided to change this particular bit of code and why he should have chosen to do so now! He stared at Jericho. 'From what you were saying, it sounds as though he couldn't have chosen any one thing more damaging to us.'

Jericho didn't have to reply; Logie did it for him. 'Routine. Almost certainly. They change their code books from time to time. Just our bad luck they did it now.'

'Routine,' repeated Wigram. 'Right.' He smiled once more. 'Tell me, Leonard, how many people know about this weather code and how important it is to us?'

'Really, Douglas,' laughed Skynner, 'whatever are you suggesting?'

'How many?'

'Guy?'

'A dozen, perhaps.'

'Couldn't make me a little list, could you?'

Logie looked to Skynner for approval. 'I, ah, well, I, ah…'

'Thanks.'

Wigram resumed his examination of the ceiling.

The silence that followed was broken by a long sigh from the admiral. 'I think I gather the sense of the meeting.' He stubbed out his cigarette and reached down beside his chair for his briefcase. He began stuffing his papers into it and his lieutenants followed suit. 'I can't pretend it's the happiest of messages to take back to the First Sea Lord.'

Hammerbeck said: 'I guess I'd better signal Washington.'

The admiral stood and immediately they all pushed back their chairs and got to their feet.

'Lieutenant Cave will act as Admiralty liaison.' He turned to Cave: 'I'd like a daily report. On second thoughts, perhaps better make that twice a day.'

Yes, sir.

'Lieutenant Kramer: you'll carry on here and keep Commander Hammerbeck informed?'

'I sure will, sir. Yes, sir.'

'So.' He pulled on his gloves. 'I suggest we reconvene this meeting as and when there are developments to report. Which hopefully will be within four days.'

At the door, the old man turned. 'It's not just one million tons of shipping and ten thousand men, you know. It's one million tons of shipping and ten thousand men every two weeks. And it's not just the convoys. It's our obligation to send supplies to Russia. It's our chances of invading Europe and driving the Nazis out. It's everything. It's the whole war.' He gave another of his wheezing laughs. 'Not that I want to put any pressure on you, Leonard.' He nodded. 'Good morning, gentlemen.'

As they mumbled their 'good morning sirs', Jericho heard Wigram say quietly to Skynner: I'll talk to you later, Leonard.'

They listened to the visitors clatter down the concrete stairs, and then to the crunch of their feet on the path outside, and suddenly the room was quiet. A mist of blue tobacco hung over the table like smoke rising after a battle.

Skynner's lips were compressed. He was humming to himself. He gathered his papers into a pile and squared off the edges with exaggerated care. For what seemed a long time, nobody spoke.

'Well,' said Skynner eventually, 'that was a triumph. Thank you, Tom. Thank you very much indeed. I'd forgotten what a tower of strength you could be. We've missed you.'

'It's my fault, Leonard,' said Logie. 'Bad briefing. Should have put him in the picture better. Sorry. Bit of a rush first thing.'

'Why don't you just get back to the Hut, Guy? In fact, why don't you all go back, and then Tom and I can have a little chat.'

'Bloody fool,' said Baxter to Jericho.

Atwood took his arm. 'Come on, Alec.'

'Well, he is. Bloody fool.'

They left.

The moment the door closed Skynner said: 'I never wanted you back.'

'Logie didn't mention that.' Jericho folded his arms to stop his hands shaking. 'He said I was needed here.'

'I never wanted you back, not because I think you're a fool – Alec's wrong about that. You're not a fool. But you're a wreck. You're ruined. You've cracked once before under pressure and you'll do it again, as your little performance just now showed. You've outlived your usefulness to us.'

Skynner was leaning his large bottom casually against the edge of the table. He was speaking in a friendly tone and if you had seen him from a distance you would have thought he was exchanging pleasantries with an old acquaintance.

'Then why am I here? I never asked to come back.'

'Logie thinks highly of you. He's the acting head of the Hut and I listen to him. And, I'll be honest, after Turing, you probably have – or, rather, had- the best reputation of any cryptanalyst on the Park. You're a little bit of history, Tom. A little bit of a legend. Bringing you back, letting you attend this morning, was a way of showing our masters how seriously we take this, ah, temporary crisis. It was a risk. But obviously I was wrong. You've lost it.'

Jericho was not a violent man. He had never hit another person, not even as a boy, and he knew it was a mercy he had avoided military service: given a rifle he would have been a menace to no one except his own side. But there was a heavy brass ashtray on the table -the sawn-off end of a six-inch shell-case, brimful of cigarette stubs – and Jericho was seriously tempted to ram it into Skynner's smug face. Skynner seemed to sense this. At any rate, he pulled his bottom off the table and began to pace the floor. This must be one of the benefits of being a madman, thought Jericho, people can never take you entirely for granted.

'It was so much simpler in the old days, wasn't it?' said Skynner. 'A country house. A handful of eccentrics. Nobody expecting very much. You potter along. And then suddenly you're sitting on the greatest secret of the war.'

'And then people like you arrive.'

'That's right, people like myself are needed, to make sure this remarkable weapon is used properly.'

'Oh is that what you do, Leonard? You make sure the weapon is used properly. I've often wondered.'

Skynner stopped smiling. He was a big man, nearly a foot taller than Jericho. He came up very close, and Jericho could smell the stale cigarette smoke and the sweat on his clothes.

'You've no conception of this place any more. No idea of the problems. The Americans, for instance. In front of whom you've just humiliated me. Us. We're negotiating a deal with the Americans that – ' He stopped himself. 'Never mind. Let's just say that when you – when you indulge yourself as you just did, you can't even conceive of the seriousness of what's at stake.' Skynner had a briefcase with a royal crest stamped on it and 'G VI R' in faded gold lettering. He slipped his papers into it and locked it with a key attached to his belt by a long chain.

'I'm going to arrange for you to be taken off cryptanalytical work and put somewhere you can't do any damage. In fact, I'm going to have you transferred out of Bletchley altogether.' He pocketed the key and patted it. 'You can't return to civilian life, of course, not until the war's over, not knowing what you know.

Still, I hear the Admiralty's on the lookout for an extra brain to work in statistics. Dull stuff, but cushy enough for a man of your… delicacy. Who knows? Perhaps you'll meet a nice girl. Someone more – how shall we say? – more suitable for you than the person I gather you were seeing.'

Jericho did try to hit him then, but not with the ashtray, only with his fist, which in retrospect was a mistake. Skynner stepped to one side with surprising grace and the blow missed and then his right hand shot out and grabbed Jericho's forearm. Skynner dug his fingers very hard into the soft muscle.

'You are an ill man, Tom. And I am stronger than you, in every way.' He increased the pressure for a second or two, then abruptly let go of the arm. 'Now get out of my sight.'

5

God, but he was tired. Exhaustion stalked him like a living thing, clutching at his legs, squatting on his sagging shoulders. Jericho leaned against the outside wall of A-Block, rested his cheek against the smooth, damp concrete, and waited for his pulse to return to normal.

What had he done?

He needed to lie down. He needed to find some hole to crawl into and get some rest. Like a drunk searching for his keys, he felt first in one pocket and then another and finally pulled out the billeting chit and squinted at it. Albion Street? Where was that? He had a vague memory. He would know it when he saw it.

He pushed himself away from the wall and began to make his way, carefully, away from the lake towards the road that led to the main gate. A small, black car was parked about ten yards ahead and as he came closer the driver's door opened and a figure in a blue uniform appeared.

'Mr Jericho!'

Jericho stared with surprise. It was one of the Americans. 'Lieutenant Kramer?'

'Hi. Going home? Can I give you a ride?'

Thank you. No. Really, it's only a short walk.'

Aw, come on.' Kramer patted the roof of the car. 'I just got her. It'd be my pleasure. Come on.'

Jericho was about to decline again, but then he felt his legs begin to crumple.

'Whoa there, feller.' Kramer sprang forward and took his arm. 'You're all in. Long night, I guess?'

Jericho allowed himself to be guided to the passenger door and pushed into the front seat. The interior of the little car was cold and smelled as if it hadn't been used in a long while. Jericho guessed it must have been someone's pride and joy until petrol rationing forced it off the road. The chassis rocked as Kramer clambered in the other side and slammed the door.

'Not many people around here run their own cars.' Jericho's voice sounded oddly in his ears, as if from a distance. 'You have trouble getting fuel?'

'No, sir.' Kramer pressed the starter button and the engine rattled into life. 'You know us. We can get as much as we want.'

The car was carefully inspected at the main gate. The barrier rose and they headed out, past the canteen and the assembly hall, towards the end of Wilton Avenue.

'Which way?'

'Left, I think.'

Kramer flicked out one of the little amber indicators and they turned into the lane that led down to the town. His face was handsome – boyish and square-cut, with a faded tan that suggested service overseas. He was about twenty-five and looked formidably fit.

'I guess I'd like to thank you for that.'

'Thank me?'

'At the conference. You told the truth when the others all talked bull-shit. "Four days" – Jesus!'

'They were just being loyal.'

'Loyal? Come on, Tom. D'you mind If I call you Tom? I'm Jimmy, by the way. They'd been fixed.'

'I don't think this is a conversation we should be having…' The dizziness had passed and in the clarity that always followed it occurred to Jericho that the American must have been waiting for him to emerge from the meeting. 'This will do fine, thank you.'

'Really? But we've hardly gone any distance.'

'Please, just pull over.'

Kramer swerved into the kerb beside a row of small cottages, braked and turned off the engine.

'Listen, will you, Tom, just a minute? The Germans brought in Shark three months after Pearl Harbor -'

'Look -'

'Relax. Nobody's listening.' This was true. The lane was deserted. 'Three months after Pearl Harbor, and suddenly we're losing ships like we're going our of business. But nobody tells us why. After all, we're the new boys around here – we just route the convoys the way London tells us. Finally, it's getting so bad, we ask you guys what's happened to all this great intelligence you used to have.' He jabbed his ringer at Jericho. 'Only then are we told about Shark.'

'I can't listen to this,' said Jericho. He tried to open the door but Kramer leaned across and seized the handle.

'I'm not trying to poison your mind against your own people. I'm just trying to tell you what's going on here. After we were told about Shark last year, we started to do some checking. Fast. And eventually, after one hell of a fight, we began to get some figures. D'you know how many bombes you guys had by the end of last summer? This is after two years of manufacture?'

Jericho was staring straight ahead. 'I wouldn't be privy to information like that.'

'Fifty! And d'you know how many our people in Washington said they could build within four months; Three hundred and sixty!'

'Well, build them, then,' said Jericho, irritably, 'if you're so bloody marvellous.'

'Oh no,' said Kramer. 'You don't understand. That's not allowed. Enigma is a British baby. Official. Any change in status has to be negotiated.'

'Is it being negotiated?'

'In Washington. Right now. That's where your Mr Turing is. In the meantime, we just have to take whatever you give us.'

'But that's absurd. Why not just build the bombes anyway?'

'Come on, Tom. Think about it for a minute. You have all the intercept stations over here. You have all the raw material. We're three thousand miles away. Damn hard to pick up Magdeburg from Florida. And what's the point in having three hundred and sixty bombes if there's nothing to put in them?'

Jericho shut his eye sand saw Skynner's flushed face, heard his rumbling voice: 'You've no conception of this place any more… We're negotiating a deal with the Americans… You can't even conceive of the seriousness…' Now, at least, he understood the reason for Skynner's anger. His little empire, so painfully put together, brick by bureaucratic brick, was mortally threatened by Shark. But the threat came not from Berlin. It came from Washington.

'Don't get me wrong,' Kramer was saying, 'I've been here a month and I think what you've all achieved is astounding. Brilliant. And nobody on our side is talking about a takeover. But it can't go on like this. Not enough bombes. Not enough typewriters. Those huts. Christ! "Was it dangerous in the war, Daddy?" "Sure was, I damned near froze to death." Did you know the whole operation almost stopped one time because you ran out of coloured pencils? I mean, what are we saying here? That men have to die because you don't have enough pencils?'

Jericho felt too tired to argue. Besides, he knew enough to know it was true: all true. He remembered a night, eighteen months ago, when he'd been asked to keep an eye open for strangers at the Shoulder of Mutton, standing near the door in the blackout, sipping halves of shandy, while Turing, Welchman and a couple of the other big chiefs met in a room upstairs and wrote a joint letter to Churchill. Exactly the same story: not enough clerks, not enough typists, the factory at Letchworth that made the bombes – it used to make cash registers, of all things – short of parts, short of manpower… There'd been one hell of a row when Churchill got the letter – a tantrum in Downing Street, careers broken, machinery shaken up – and things had improved, for a while. But Bletchley was a greedy child. Its appetite grew with the feeding. 'Nervos belli, pecuniam infinitam.' Or, as Baxter had put it, more prosaically, it all comes down to money in the end. The Poles had had to give Enigma to the British. Now the Brits would have to share it with the Yanks.

'I can't have anything to do with this. I've got to get some sleep. Thanks for the lift.'

He reached for the handle and this time Kramer made no attempt to stop him. He was halfway out of the door when Kramer said: 'I heard you lost your old man in the last war.'

Jericho froze. 'Who told you that?'

'I forget. Does it matter?'

'No. It's not a secret.' Jericho massaged his forehead. He had a filthy headache coming on. 'It happened before I was born. He was wounded by a shell at Ypres. He lived on for a bit but he wasn't much use after that.

He never came out of hospital. He died when I was six.'

'What did he do? Before he got hit?'

'He was a mathematician.'

There was a moment's silence.

'I'll see you around,' said Jericho. He got out of the car.

'My brother died,' said Kramer suddenly. 'One of the first. He was in the Merchant Marine. Liberty ships.'

Of course, thought Jericho.

'This was during the Shark blackout, I suppose?'

'You got it.' Kramer looked bleak, then forced a smile. 'Let's keep in touch, Tom. Anything I can do for you – just ask.'

He reached over and pulled the door shut with a bang. Jericho stood alone on the roadside and watched as Kramer executed a rapid U-turn. The car backfired, then headed at speed up the hill towards the Park, leaving a little puff of dirty smoke hanging in the morning air.