The memo landed on Kim Philby's desk in Washington, DC, in July 1950. Three months later, Bruno Pontecorvo, a physicist at Harwell, Britain's atomic energy lab, disappeared without a trace. When he re-surfaced six years later, he was on the other side of the Iron Curtain.One of the most brilliant scientists of his generation, Pontecorvo seemed to have been privy to many secrets: he had worked on the Anglo-Canadian arm of the Manhattan Project, and quietly discovered a way to find the uranium coveted by nuclear powers. Yet when he disappeared MI5 insisted he was not a threat. Now, based on unprecedented access to archives, letters and surviving family members and scientists, award-winning writer and physics professor Frank Close pieces together an answer to whether Pontecorvo's defection ended a life of spycraft – and exposes a life irrevocably marked by the advent of the atomic age and the Cold War.