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Summer 1941. Klaus Fuchs enters an unremarkable red-brick building in Birmingham, makes his way down a long grimy corridor, and pauses in front of double doors that bear the sign: TUBE ALLOYS. It is the codename for the British atomic bomb project. This is where the fate of the world is being decided.

Fuchs has been taken under the wing of fellow German émigré and scientist, Rudolf Peierls, who desperately needs Fuchs’ genius in the race to build the bomb. Fuchs lodges at Peierls and his wife’s house and the trio grow close. Little do the Peierls know, they have a Soviet spy under their roof.

As Fuchs works tirelessly on the British atomic bomb project, he is simultaneously slipping secrets to the Soviets, so that they can build their own bomb, playing an intrinsic role in both sides of the atomic race. But Fuchs doesn’t work alone. He soon meets his new handler, a Soviet spymaster who just so happens to be presenter Rosa Ellis’ great-aunt, Ursula Kuczynski. Fuchs knows her only as Agent Sonya. Together, the pair of them are the most important spies on the planet. This is only the beginning of a partnership that will dramatically alter the course of history.

Fuchs and Ursula have been given a list of twelve pieces of information that Russia urgently needs. But as the British nuclear project ramps up, trouble is brewing for Fuchs. Will his past get in the way of his mission?

In 1943, Fuchs and Peierls are recruited for the new US atomic effort. Security on the Manhattan Project is tighter than it is in Britain. Suspicions are raised about Fuchs, and his double life suddenly becomes even more dangerous.