When Susan Gray spots a hidden pattern in a series of murders, she tries to inform the police. But when they and her husband insist she forget her theory Susan enlists her wartime friends to try and track down the murderer.

As Susan and the other Bletchley women close in on the murderer of the five girls, a shocking discovery calls their theories into question and forces them to set a dangerous trap - a trap that puts one of them in even greater danger.