That jolted George. 'Really? What's this based on?'

She grinned. 'A good historian's question. Very little. Grave markers, that sort of thing, and a lot of speculation. But Christ had died less than a century earlier. Given that, it's at least plausible that Tullio would have had in his possession some sort of relic of the Crucifixion. A soldier's trophy. A spear, perhaps – like the Spear of Longinus, said to have been used to wound the dying Christ.'

'Ah. Hitler's got that, hasn't he? And Himmler and his crew are always on the look-out for holy relics.'

'Yep. But if Tullio did have such a spear, and he was stationed at the Wall, and his descendants stayed on after him-'

'It might have finished up buried at Birdoswald. Only to be dug up by a Navy bloke two thousand years later.'

'That's the general idea.'

'This is an utter pack of lies, isn't it, Mary?'

'Absolutely. But Himmler's toadies have been fooled by much less convincing frauds.'

The photographers took more snaps, of the party as a whole under the spear, then of Josef Trojan and Mary together, and Trojan alone. He had the photographers crouch, so they looked up at his handsome face, his crossed arms, the spear on the wall behind him.

Mary watched this, fretting, light-headed. Knowing what was to follow, she felt furiously impatient with this buffoon and his show-boating, even though she knew it was that quality about him that had got her in here in the first place.

At last Trojan was done. He straightened his uniform and approached Mary again. 'This day is perfect, for me – perfect. And now let me make it perfect for you, my dear Mary Wooler.' He turned and nodded to his brother, who, looking a bit embarrassed by all the song and dance, opened a door to a staff room.

And Gary walked out. There was a young woman as his side. He wore a smart suit and tie, his shoes were polished, his hair cut and combed.

Mary ran to her son and grabbed him. She hadn't seen him since the invasion, since his capture. Gary hugged her back, hard; she felt his strength, an echo of his father's. Flashbulbs popped. Trojan and the other Germans applauded. Mary ignored them all.

Gary drew back and held her arms. 'Mom. Hey, no tears. You'll mess up the suit.'

'I wasn't expecting- when I thought of you, I imagined you in your uniform. Seeing you in a suit, it's as if the war never happened.'

'Yeah, but my uniform don't look so good after a year in the stalag, believe me.'

She looked at him intently. 'And you're well?'

He shrugged. 'Life in Rutupiae is pretty good. Their doctors gave me a good check-over. Compared to the stalag, I'm well off.'

'And of course he has me.' The young woman approached tentatively. She was dressed in white, attractively if soberly; her face was square, sensible, and quite well made up. This was Doris Keeler, and one eye flickered, a subtle wink at Mary, her old friend from Colchester. 'Sophie Silver,' she said boldly. She held up her left hand and waggled her ring finger. It bore a band of silver. 'Though, with your blessing, it will soon be Mrs Sophie Wooler. And hopefully before any little Aryans come along.'

'Oh, my dear.' Mary embraced her. 'It's so good to meet you. Gary told me all about you in his mail.'

'All good, I hope.' She plucked at Gary's lapel. 'Though he didn't listen to me about this suit. A bit spiwy, don't you think?'

More flashbulbs popped as they talked. Gary Wooler, American veteran of the invasion, was now the poster boy of Himmler's crackpot breeding programme. It had all been perfect, Mary thought, immaculately set up by MI-14 and the resistance – a trap of publicity and achievement this vain, ambitious Josef Trojan couldn't resist falling into.

But Julia Fiveash was staring intently at them, her eyes narrow, her face expressionless. Trojan might have taken the bait, Mary reminded herself, but he wasn't the only player here.

Delicately Trojan approached them. 'I am delighted to have been the agent of this long overdue reunion,' he said. 'But now I must ask you to come with me. Mrs Wooler, I have a number of scholars eager to discuss with you their work on the implications for the medieval age of the Norman Conquest – I know that is an interest of yours. Many of our scholars are English, you know – indeed many have come into the protectorate specifically to work at this institute! And we have good links with several English universities, including both Oxford and Cambridge. As you remarked, there are bonds of scholarship which, eternal, transcend the petty political squabbles of the day. And then you must let me treat you to lunch…'

He walked on with Mary, with George, Doris and Gary following, the rest of the German staff and the patient photographers, on deeper into the bowels of his brand-new college, their footsteps echoing on the granite floor tiles.

The space under the floor was only about three feet deep, and full of water and gas pipes and electricity cables. Gary had to climb through this jungle from joist to joist, ducking past the pipes and cables, fearful that at any moment he would put his foot through the basement ceiling under him and ruin everything.

Doris seemed able to squirm through it all with remarkable ease. By the light of the torch strapped to his head, Gary saw her wriggling away ahead of him, her legs bare save for her stockings, her skirt tucked up without self-consciousness into her belt. She wasn't even getting her white outfit dirty. 'Come on, keep up,' she whispered back at him.

'I haven't had your training.'

'What about all that tunnelling out of the POW camp?'

'That was for the English,' he said. 'The public school types. Anyhow the Germans kept a close eye on me. A Prominente, remember.'

'What a rotten excuse. We're close, I think.'

It hadn't been hard to slip away from his mother's group and out of sight of the various German guards. Once they were alone, in a kind of reading room, Doris had shown him the diagram the resistance spies had assembled of this Ahnenerbe facility. They knew that Ben was being held in a kind of laboratory tucked away in the basement. 'Of course it would be the basement,' Gary had remarked. 'Nazis like basements.' It had taken Doris only minutes to lift the carpet, prise up a couple of floorboards, and slip down into the space between the ground level floor and the basement ceiling.

'Here.' She came to a stop. With care she unscrewed a light fitting, pulled it back, and peered through the hole in the ceiling plaster. 'Bingo. And there's nobody around. Probably all watching the show upstairs…' She took a knife from under her skirt and briskly cut a circle in the plaster, a couple of feet across. She looked down again. 'Only six or eight feet. Piece of cake.' She grabbed a wooden joist and swung her feet down through the hole. She dangled by her arms from the joist. Then she let go and dropped, bending her legs so she landed without impact, and virtually no noise.

Gary came to the hole. The room below was brightly lit. He glimpsed mechanical equipment, a glass wall. Doris stood directly beneath him. He could see plaster dust on her hair. 'Now you.'

He landed heavily, with a noisy clatter, and nearly stumbled over.

'Idiot,' she hissed.

'Show-off.' He straightened up, brushing the dust from his suit jacket, and looked around. The room was a box, brightly lit, the walls whitewashed. The central area was walled off by glass, a room within a room. There were desks, work tables and chairs, mounds of paper heaped up – and, incongruously, a big bookcase that contained mouldering history titles. There was a hum of fans; the air was dry, cool.

But the place was dominated by a bank of mechanical gadgetry that covered one wall, side to side, floor to ceiling. It was as he imagined a telephone exchange might be, all relays and wires in an aluminium frame.