Spring, 1941. The struggle of the war continues with grim news of defeats abroad and relentless bombing raids at home. A breakthrough is needed at Bletchley Park where desperate efforts are being made to crack the German Enigma code. This endeavour is suddenly very close to home for Frederick Rowlands, blind veteran of the Great War, when his cryptographer daughter, Margaret, 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.