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It is 1949. British intelligence have both Klaus Fuchs and Ursula Kuczynski caught in their crosshairs. They put their top interrogator, Jim Skardon, on the case. Could this be the end for history’s most dangerous spy?

Meanwhile, Fuchs is oblivious. He’s left espionage behind, is focusing on academia and enjoying life in England, a country he has, despite everything, come to call his home. He’s just been offered a potentially life-changing opportunity at the University of Leipzig, in communist East Germany.

Unfortunately for Fuchs, this provides the perfect excuse for Skardon to size him up.

Will Skardon get Fuchs to finally come clean? Will Fuchs reveal himself to his closest friends, the Peierls, who took him in and trusted him enough to let him stay under their roof, all whilst he was slipping nuclear secrets to the Soviet Union? Or will he remain loyal to the cause, and die with his lies?

Presenter Rosa Ellis grew up hearing stories about her great aunt, Ursula Kuczynski, aka Agent Sonya, Klaus Fuchs’ handler. Their partnership changed the course of history and set the stage for today’s nuclear threat

As Fuchs’ double life draws to a close, Rosa wants to know what the fallout of this human chain reaction was, and how Fuchs and Ursula’s actions are still affecting us today.