He’d recently bought several summer jackets, shirts and trousers from Hardy Amies’ new line of menswear, and picked out a suitable combination. He matched them with a pair of brown brogues from Crockett & Jones, his Omega watch, and a slim black leather wallet from Dunhill.

Dressing less conspicuously was another lesson he’d tried to teach the Watchers. Their dark suits and light mackintoshes, which had been intended to make them blend in, were starting to date and become easy to spot as austere post-war fashions started to relax. Knox had encouraged them to broaden their sartorial horizons, but this had just resulted in more resentment. It had taken the Watchers a lot of hard work to earn enough to buy their uniform, and they wore it with pride.

He took an early train to Cambridge. It wasn’t busy—most people were making their way into London, not the other way round—yet Knox still didn’t have his first-class carriage to himself. Just before the train pulled out of King’s Cross a man joined him in the small compartment. The day was already warming up and the man was sweating in his oversized three-piece suit. Even though it was summer he also carried a thick coat along with his bulging suitcase. He had the look of a travelling salesman.

Thankfully the man didn’t seem interested in making conversation and sat silently across from Knox, reading his newspaper, until the train reached Hitchin. Knox watched him drag his suitcase down onto the platform and then, as the train pulled out of the station, reached over for the paper he’d left behind. Knox scanned the headlines as the countryside rolled by. Calder Hall, which had been front-page news a few days ago, was already relegated to a single column on page five, replaced by a large, close-up photo of Yuri Gagarin sipping champagne. Knox, happily, couldn’t find any mention of Holland, Manning, or himself anywhere.

Once he reached Cambridge, Knox headed to the Cavendish Laboratory on Free School Lane, home to the university’s physics department. The Cavendish was a warren of narrow passages, dead ends and staircases. He passed a steady stream of postgraduate students—some, he thought, more than once—who all shuffled quietly around him, their eyes glazed over from a summer buried in experiments and books.

Eventually, he stopped one and asked for directions. Two staircases and more corridors later he found the door with a little brass plaque next to it that read:

Dr Ludvig Kaspar, Emeritus.

Kaspar was German, and a polymath. He was a prodigious mind who had made a name for his young self across Europe by being able to take on almost any problem in any scientific field. Then the war came, and the Nazis put him to work on a task that in Kaspar’s mind was entirely beneath him—improving the guidance systems of their V2 rockets.