Set in 1952, following four women whose work at Bletchley Park during WWII helped break codes used by the German military. Susan, Millie, Jean and Lucy have returned to their normal lives, but when Susan identifies a hidden pattern in a series of murders, she enlists the help of her former colleagues to crack the case.

The women close in on the killer of the five girls, narrowing their list of suspects down to three. However, a shocking discovery forces them to change their theories and they set a trap to catch the culprit using Lucy as bait - but the operation goes horribly wrong.

Susan and her code-breaking allies seem to have finally tracked down the murderer, and attempt to confront him. As the women get closer to their target, they are forced to put themselves in grave danger.

Former Bletchley Park girl Alice Merren is in prison awaiting trial for the murder of the eminent scientist John Richards. Driven by her belief in Alice's innocence and that she is hiding the truth about John's death, Jean attempts to reunite the circle to help establish the innocence of one of their own.

Following the discovery of Lizzie Lancaster's true identity the women are faced with a race against time to prove Alice's innocence and to save their friend from the hangman's noose. However further investigations lead them to uncover a much darker military conspiracy.

Millie has been kidnapped and it appears that it could be connected to her recent involvement in the 'import/export' business. Worried about her friend, but not wanting to incriminate her by contacting the police, Alice contacts the other women to help find their friend. But when they go to search for Millie's business associate Jasper, it becomes clear that Millie has become embroiled in a very dangerous world.