'The Post Office?'

'They are used to handling this kind of equipment in their telephone exchanges. Look here, Ernst – see the valves, the glass tubes? We have over two thousand in this machine alone, each capable of switching from one state to another in just a millionth of a second. It is this speed of switching, you see, which enables the machine to carry through its computations so rapidly.'

'And how do you express your problems to it?'

'Ah, good question. You speak" to this beast in a physical language. It is a question of setting switches and plugging in cables, as if reordering its very brain. These are the most advanced thinking machines in the world! And with such devices the computation of Godel trajectories becomes trivial.'

'Trivial." You mean that in the academic sense, don't you? Not an intellectual challenge." But perhaps one should apply the word to your whole enterprise, Josef.'

Fiveash laughed. 'Your muddy foot-soldier of a brother has a brain in there, Josef.'

'Unterscharfuhrer Fiveash, yes, I am a muddy infantryman, and proud of it. That is the reality of the war to me. Mud and guns and blood, hunger and death. All this talk of ancient powers and time travel is so much claptrap. You already failed once – that nonsense over Hastings!'

'But we will not fail again.' And can you not see,' said Josef Trojan earnestly, 'that if we succeed we will transform the fortunes of the war at a stroke? For we will cut down our most powerful opponent-'

'America.'

'Yes. Cut it down at the root! Let the Americans shuffle their tanks and ships around the world; it will avail them nothing. We have a plan, you see, a new plan to do with Christopher Columbus, and the beginning of it all. Believe it or not, our historical research is taking longer than the technical. But we are making progress. And then we will see how it goes for the Reich, in a new and transformed world in which America does not exist at all.'

And nor would the Reich exist, Ben thought. You unimaginative fool.

The voices fell silent. Perhaps he had spoken aloud.

He opened his eyes. The lamps' glare dazzled him, and he blinked away tears. He could see the three of them standing just outside the glass wall of his chamber, two black SS uniforms, a uniform of the Wehrmacht. Ben tried to see the brother's face, Ernst's. He imagined himself as Ernst must see him. His skinny frame lying above the smoothed-out sheets. The shackles that bound his wrists and ankles and neck. The tubes that snaked under his striped prison-issue pyjamas and into veins in his arms and legs, into his penis and into his mouth. The metal cap that had been fixed to his scalp, attached with screws that had been tapped into the very bone of his skull.

Josef Trojan's face loomed like a moon. 'Good morning, little fellow. Or is it good afternoon? You never know, do you? Don't you have anything to say, Ernst? Remarkable sight, isn't he – a triumph for modern medical science. He never leaves his little glass room, save in the imagination, of course. He can't do anything. He can't even play with his own circumcised, cathetered cock, poor fellow. All he can do is sleep – and I control even that, for by turning this switch I can administer drugs to him at will, do you see? Sleep, and dream the dreams I command through the loudspeakers that surround his pillow. And as he sleeps he guides the teasings of the Loom of history, and by doing so he wins the war for Hitler. What do you think, Ernst? Even you must be impressed.'

'What I see here is cruelty. Arbitrary, pointless cruelty. Such ambition and vanity will bring us down in the end, Josef.'

'If you believe that you really are a fool.'

But Ben heard uncertainty in his voice. As the months had turned into years, others had expressed similar doubts over Trojan's elaborate project and the resources it was consuming. It was a long time indeed since Himmler had shown any support, let alone visited Richborough. Trojan had even once been hauled in by the Gestapo for an interrogation. The experience had left him shaken and unsure. But Julia Fiveash was always on hand to stiffen his spine.

'Enough,' Trojan said. 'Let's put him back to work.' He turned his switch. Ben felt the opiates course into his blood. The brothers and Fiveash walked on around the facility. 'You should come work with me here at Richborough, you know, Ernst,' Trojan said. 'There are Wehrmacht guards here. I'm sure I could fix a transfer.'

'My duty lies elsewhere.'

'Your trouble is you never got over that wretched French girl, did you? It's muddled your thinking. You always were a fool…'

The world spun away, as if he were tumbling down a well, and Ben, trapped inside himself, fell into fragmented dreams.

18 June

Mary's train into York was late.

When she got to the little tea shop on Low Petergate they were waiting for her, sitting at a window table, drinking tea and eating cake: Tom Mackie, slightly crumpled and donnish as always despite his Navy uniform, and Gary, his own British Army uniform fitting him closely. Both men got up when Mary squeezed her way to the table, hot, flustered, tired. 'Mom-' Gary embraced her. He smelled of cigarette smoke, earth, a whiff of cordite, a soldier's smell. But the arms around her were strong. It made her ache that she would only have a few minutes with him.

Mackie pulled out a chair. 'Good to see you, Mary. Tea, is it, a scone or two? You might have to wait a bit, I'm afraid; all these GIs are rushing the girl off her feet.' He turned and raised his arm, trying to catch the waitress's eye.

'Thank you. Sorry I'm late.' She sat, setting her handbag and gas-mask pouch on the floor beside her.

The shop was crowded with servicemen, talking loudly, smoking, most of them apparently American. And music played, a sentimental Glenn Miller ballad. It was probably the Promi; you heard it played everywhere for the music, which everybody agreed was a better selection than the BBC's. She was right in the big picture window; it was a surprise such a window had survived the bombing. Looking along a street crowded with military vehicles and black government saloon cars, she could just make out the angular ruins of the minster.

'You look a bit hot and bothered,' Gary said.

'The train's a trial at the moment, isn't it? Packed with servicemen, en route from A to B.'

'You do get a sense of the mobilisation, don't you?' Mackie said. 'Lots of pieces being moved around the board, ready for the chess game to begin.'

It was all hush-hush, but it was impossible not to know what was going on. Troops had been pouring into the country for months. In the north and in Scotland farms and villages had been evacuated, vast tracts of land set aside for training. It was said that the US Eighth Air Force had pretty much taken over East Anglia. At night, across a vast swathe of countryside north of the Winston Line, you could hear the rumble of tanks and mobile weapons, of studebakers and jeeps, trundling to their marshalling points under cover of darkness.

And here was her own son, preparing to throw himself into the cauldron. He seemed so much less boyish than when she'd last seen him. Now he had a man's heavy body, a thickening neck, even slightly thinning hair like his father's. And he was full of nervous energy. He kept glancing at a clock on the wall.

'You've filled out,' she said to him.

'Well, I'd hope so,' he said patiently. 'Mom, it's eighteen months already since I got sprung from that lebensborn camp in Kent. They do give us decent rations, you know.'

Mackie tamped his pipe tobacco. 'Better in the US Army, I hear. You could always swap sides.'

Gary shook his head firmly. 'I started this thing in these colours, and I'll finish it in them.'