Germany 1941. Two British officers, Hugh Hartley and Malcolm Royce, achieved what many believed to be impossible. They escaped from Oflag IVC, better known as Colditz Castle. But as they are about to cross the border into Switzerland, and within yards of reaching freedom, Royce is shot. He begs Hartley to go on and save himself. Wracked with guilt, Hartley leaves his friend behind.

London, 1973. Thirty years later and Hartley is now a senior MI6 officer. When a shadowy contact tips him off that Royce may still be alive, and still being held in Colditz - now a lunatic asylum - Hartley is desperate to discover what really happened to his friend. He plans a perilous mission to break back into Colditz, but the truth he will find there will be more shocking than he could possibly have imagined.