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.