One man's mission started the Cold War. Superspy unearths how Klaus Fuchs stole the secrets of the Hiroshima bomb and gave these confidential details to the Soviet Union.

During the Second World War, German refugee Klaus was posted to the highest security weapons laboratory in America. His assignment was to help design the world's first weapon of mass destruction. After joining Robert Oppenheimer's team, he became an expert on plutonium and secretly plotted how to contact the Soviet spymasters. Under the eyes of the FBI, he slowly pieced together America's atomic secrets and copied out his notes. Evading security, he smuggled out the complete blueprint of the Nagasaki A-bomb.

In January 1942, Klaus met up with a young mother – who was, in fact, a Soviet spy – and disclosed the classified information of how to construct an A-bomb. In the spring of 1945, he conducted a series of meetings with his Soviet courier, "Harry Gold". By 1949, the FBI were on the hunt for the traitor. Klaus escaped to England, where he started a job which placed him at the heart of the British nuclear establishment. While in the UK, he continued to sell secrets. The superspy's downfall came when he confessed all to MI5, whom he told: "It's as though my mind has two compartments." But the consequences of his actions led the world to fear nuclear Armageddon.