At the end of the war, Kaspar, who had been brought to Berlin to avoid him falling into enemy hands, fell into enemy hands. He had been high on the GRU’s list of prizes, and when Russian soldiers found him working in his temporary lab as if the Third Reich wasn’t crumbling outside, he was transported to Moscow, where he was thoroughly and relentlessly debriefed.

Once the GRU had learned all they could from him to help them advance their own intercontinental ballistic missile programme, he was allowed to return to Berlin. Kaspar hoped to find his lab still intact. It wasn’t. Deep inside the Soviet-controlled eastern sector of the city, the building had long since been repurposed and all traces of his lab gone. He wrote to several universities in the Bundesrepublik to the west seeking a position, but none of them wanted anything to do with someone now considered a Nazi and Soviet collaborator.

The CIA, however, did. He was approached, and then spirited out of East Berlin to Washington as part of Operation Dragon Return. When the US had extracted everything of value from him about both the Soviet Union’s missile programme and life behind the Iron Curtain, the British were given their chance.

Kaspar had been a brilliant young scientist, but after almost ten years being traded between superpowers and endlessly debriefed, the light of his genius had become dim and jaded. There was little flesh on his intellectual bones for the British to pick on. But there was also nowhere else for Kaspar to go. He was almost sixty and had no family, and no desire to return to a Germany that didn’t want him back.

In a fit of generosity, he was given rooms and a small office in Cambridge where he could live out his dotage quietly. In the last six years he’d given two lectures, had published one short paper, and had generally been ignored by successive generations of students passing through the university. It was a stroke of luck that the postgraduate Knox stopped for directions knew about ‘the old Nazi in the rafters’.

Knox tapped on Kaspar’s door. There was no response. He knocked again, louder. Still no acknowledgement or movement inside. Thinking his luck might already have run out, Knox was about to give the door one last bang when it was suddenly pulled open by a short, flushed woman in a thin summer jumper and black capri pants.

‘What do you want?’ she demanded, in a strong Midwestern American accent. She looked very young, but that could have been her height, close-cropped dark hair, and flushed cheeks conspiring against her.

‘I’m looking for Dr Kaspar,’ Knox replied.