September 1939. London is in blackout, war has been declared, but Dulcie Treadwell can think only of American broadcaster Glenn Reeves, who left for Berlin without being able to say goodbye. Posted to Bletchley Park, Dulcie must concentrate instead on helping the war effort by cracking the German Enigma codes. The hours are long and the conditions tough, with little recognition from above. Until she breaks her first code... Dulcie finally settles into something she feels born to do. But just as she's proving her worth, a spiteful act of jealousy leads to Dulcie's brutal dismissal. Is it too late for Dulcie to clear her name and keep saving innocent lives through the job she loves?