The Cambridge Spies continue to fascinate - but one of them, John Cairncross, has always been more of an enigma than the others. He worked alone and was driven by his hostility to Fascism rather than the promotion of Communism. During his wartime work at Bletchley Park, he passed documents to the Soviets which helped them to win a decisive victory over Nazi Germany at the Battle of Kursk. Now, Geoff Andrews has accessed the Cairncross papers and secrets, and spoken to friends, relatives and former colleagues. A complex individual emerges - a scholar as well as a spy - who motivations have often been misunderstood. The full account of Cairncross's spying, his confession and his dramatic public exposure at the 'fifth man' will be told here for the first time, while also unveiling the story of his post-espionage life.