The world first heard of Klaus Fuchs, the head of theoretical physics at the British Research Establishment at Harwell, in February 1950 when he appeared at the Old Bailey, accused of passing secrets to the Soviet Union. For over 60 years the Governments of Britain, the United States and Russia tried to cover up the truth about his treachery. Here, Mike Rossiter unravels the truth about his clandestine meetings, deadly secrets, family entanglements and illicit love affairs, all set against the tumultuous years from the rise of Hitler to the start of the Cold War.