Spirited Millie is earning a bit of money selling odds and ends on the black market. But Millie stumbles across a seam of post-Second World War unwholesomeness when she annoys some very dangerous people and gets into trouble. Soon, the other Bletchley girls must rally to her aid.

Former Bletchley Park worker 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 regroup the circle in order to help establish the innocence of one of their own.

Following the discovery of Lizzie Lancaster's true identity, the women face a race against time to prove Alice's innocence and save her from the hangman's noose. However, subsequent investigations lead to the discovery of a much darker military conspiracy.

Lack of action by the police leaves the women to conclude there must be corruption within the vice squad and that catching the gang red-handed is their only option. Plotting to infiltrate the crime ring to find out the location of the hotel being used as a base, they visit Marta in her restaurant, pretending Jean is a potential new client. This leads to an opportunity for Lucy to memorise the businesswoman's ledger, which may hold the key to the whole operation.

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.

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.