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It is 1944. Russia and America are racing for the bomb, but neither competitor knows who is in the lead.

Klaus Fuchs arrives in Los Alamos to begin work on the Manhattan Project. He is closer to the action than ever before. But security here is higher than it was back in England and Fuchs is frustrated to find that the secrets the Soviets need are still out of reach.

As the pressure builds, Fuchs must put his skills in both science and espionage to the test. In the process, he’ll put his own family in danger.

In 1946, Fuchs returns to Britain a man very different to the one that left; older, more confident, and an indispensable part of the British scientific establishment. He begins to settle down, find friends, and make a life for himself. Little by little, he leaves espionage behind.

But all the while, the British intelligence services are closing in on both him and his handler, Ursula Kuczynski, presenter Rosa Ellis’ great aunt. Top interrogator, Jim Skardon, is tasked with interrogating Ursula. It’s a battle of wits that will put Fuchs at risk of being exposed.