In September 1945 Igor Gouzenko, a cipher clerk based at Ottawa's Soviet embassy, traded a batch of purloined files in return for a secure future for himself, his pregnant wife and children. His defection led to the slow unravelling of Soviet efforts to steal American atomic secrets, which eventually led to the arrest of Dr Allan Nunn May, a Cambridge-educated physicist who confessed to having passed atomic secrets to a Russian contact in Canada in 1944. Critically acclaimed author Nigel West has been given unprecedented access to files in the British and US archives relating to the Soviet espionage apparatus. His research reveals exactly how the USSR was able to steal and use the technological secrets. Their success led to the building of the Soviet atomic bomb. The identities of Dr Nunn May's colleagues on the Manhattan Project who collaborated with the NKVD have never been publicly revealed. West is the first writer to tie together all the ramifications of the Soviet spying effort to steal the A-bomb. The effort stretched across many countries, taking in accomplices and agents in the UK, Canada and the United States in a tightly constructed demonstration of the effectiveness of the NKVD and GRU methods. The decrypted Venona cables are reproduced and juxtaposed with the replies given by the Moscow centre, showing the full circle of the espionage effort.