The note arrived at Kim Philby's office in Washington in July 1950. Three months later, Bruno Pontecorvo, a physicist at Harwell, a British atomic energy laboratory, disappeared without a trace. He would reappear six years later on the other side of the Iron Curtain. One of the brightest scientific minds of his generation, Pontecorvo was privy to many secrets: he had worked on the Anglo-Canadian branch of the Manhattan Project and had discovered a way to locate the uranium that was in such high demand by emerging nuclear powers. However, when he disappeared, MI5 insisted that he was not a threat. Based on access to archives and letters, interviews with members of his family and the scientists he collaborated with, award-winning author and physics professor Frank Close exposes the truth about a man who was closely connected to the advent of the atomic age and the Cold War.