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Who was Klaus Fuchs, the man who stole the bomb? How did a prodigious young talent at the beginning of a promising academic career, evolve into a fully committed Soviet spy?

Journalist Rosa Ellis grew up hearing stories about her great aunt, Ursula Kuczynski, aka Agent Sonya, Klaus Fuchs’ handler. How did Fuchs and her aunt come to cross paths?

Fuchs comes of age in Nazi Germany, but his scientific brilliance offers him an escape - to England. An ‘enemy alien’ with Communist leanings, he is deemed as a risk to the country and shipped off to Canada, where he falls into the Soviet espionage web. Upon his return to England, his double life begins. At the same time, another German emigre and scientist, Rudolf Peierls, seeks a trustworthy assistant and a brilliant scientist for the burgeoning British atomic bomb project. So begins a human chain reaction with devastating potential.

Fuchs soon finds himself in the belly of the beast, assisting Peierls on the bomb whilst simultaneously slipping secrets to Agent Sonya, aka Ursula Kuczynski, Rosa’s great aunt and the Red Army’s leading intelligence officer in Britain.

Fuchs and Peierls’ friendship grows stronger and the two are quickly snapped up for the new US atomic effort. Suspicions are raised about Fuchs and his double life suddenly becomes even more dangerous, as his espionage skills are put to the test.

Finally MI5 begin to close in on both Fuchs and Ursula, and top interrogator Skardon pushes their loyalties to the limit. Will Fuchs reveal himself to his closest friends, the Peierls, who trusted him enough to let him stay under their roof? Or will he remain loyal to the cause, and die with his secrets?

This programme has been edited for accuracy. Audio was updated on 20th July 2022

(Photo: Klaus Fuchs (1911-1988), German physicist and spy. Credit: Getty Images)