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.

Thrown into a chilling confrontation with the murderer they have so brilliantly tracked down, Susan and the other Bletchley women become locked into a ever more dangerous game of cat and mouse that leads to a seemingly inescapable conclusion.