John Cairncross was among the most damaging spies of the twentieth century. A member of the infamous Cambridge Ring of Five, he leaked highly sensitive documents from Bletchley Park, MI6 and the Treasury to the Soviet Union – including the first atomic secrets and raw decrypts from Enigma and Tunny that influenced the outcome of the Battle of Kursk. In 2014, Cairncross appeared as a secondary, though key, character in the biopic of Alan Turing’s life, The Imitation Game. While the other members of the Cambridge Ring of Five have been the subject of extensive biographical study, Cairncross has largely been overlooked by both academic and popular writers. Despite clear interest, he has remained a mystery – until now.