London, 1940. Sheila Phipps is orphaned after a devastating air raid claims her family and her home. She is evacuated to Bletchley to live with her aunt Constance, where she forms an unlikely friendship with Prudence Le Strange, who is working in the code-breaking unit at Bletchley. As their friendship grows stronger, the war subjects Sheila and Prue to fresh tragedies as, one by one, those they love are called away to distant battlefields, only to join the growing ranks of the missing, the captured and the dead. As the war escalates, the two friends find their lives increasingly complicated not only by the secrets of wartime but by those the conflict has dredged up from the past.