German by birth, British by naturalization, Communist by conviction, Klaus Fuchs was a fearless Nazi resister, a brilliant scientist, and an infamous spy. He was convicted of espionage by Britain in 1950 for handing over the designs of the plutonium bomb to the Russians, and has gone down in history as one of the most dangerous agents in history. He put an end to America's nuclear hegemony and single-handedly heated up the Cold War. But, was Klaus Fuchs really evil? Using archives long hidden in Germany as well as intimate family correspondence, Nancy Thorndike Greenspan brings into sharp focus the moral and political ambiguity of the times in which Fuchs lived and the ideals with which he struggled. With thrilling detail from never-before-seen sources, Atomic Spy shows the real Klaus Fuchs—who he was, what he did, why he did it, and how he was caught. His extraordinary life is a cautionary tale about the ambiguity of morality and loyalty, as pertinent today as in the 1940s.