Shortly after the end of the Second World War, on 5 September 1945, an event occurred which would allegedly mark the beginning of the Cold War. A young cipher clerk working for the GRU, Soviet military intelligence, defected to the West, bringing with him a raft of documents and exposing a large spy network on both sides of the Atlantic. At first, the Canadian Prime Minister, Mackenzie King, was loathe to react, fearing that any action taken by his government might spark a Third World War with the Soviets.  Igor Gouzenko revealed that a British nuclear physicist, Alan Nunn May, who had worked on the Tube Alloys/Manhattan Project, was part of that spy network and had passed on secrets about the atomic bomb to the Russians. It would cause a rift in US-British relations and lead to the so-called ‘McMahon Act’ whereby Britain, and therefore Canada, were excluded from further collaboration in developing atomic research. It also led to a Canadian Royal Commission to investigate the extent to which spies had penetrated the Canadian government and what information they had passed on. Gouzenko and his family lived under witness protection in fear of assassination from the KGB, while Alan Nunn May, on his return to the UK, was tried and convicted of espionage. When he was released from prison in 1952, he had problems finding employment. When he died, he still firmly believed that what he had done had been justified. This book examines the ‘Gouzenko Affair’ and corrects some of the myths surrounding his story. It also considers the case against Nunn May, his associates, and his life as a convicted but unapologetic spy.