D-Day, 6 June 1944, the turning point of the Second World War, was a victory of arms. But it was also a triumph for a different kind of operation: one of deceit, aimed at convincing the Nazis that Calais and Norway, not Normandy, were the targets of the 150,000-strong invasion force. The deception involved every branch of Allied wartime intelligence - but at its heart was the 'Double Cross System', a team of double agents controlled by the secret Twenty Committee. These double agents were, variously, brave, treacherous, fickle, greedy, and inspired. They were not conventional warriors, but their masterpiece of deceit, never before fully revealed, saved countless lives. Their codenames were Bronx, Brutus, Treasure, Tricycle and Garbo. This is their story.