Spring, 1941. The Second World War has entered a dangerous phase, with British ships being torpedoed in the Atlantic and nightly bombing raids on major ports. At Bletchley Park, top secret home of the nation's code-breakers, the race is on to crack the German Enigma code and thus prevent further naval and military losses. This endeavour is suddenly very close to home for Frederick Rowlands, blind veteran of the Great War, when his daughter, Margaret, who works at 'the Park' as a cryptographer, is arrested on suspicion of betraying secrets to the enemy.

Then a young woman is found murdered, and Rowlands is drawn into a deadly battle of wits where he must decode a series of clues that will lead him to the killer and enable him to discover the real traitor at Bletchley Park.