During the war Alice Merren was one of Bletchley's brightest code-breakers. Now it is 1953. Lucy has a clerical job for Scotland Yard, Millie works as a German translator and Susan a housewife perturbed that her husband's promotion may take them overseas. Jean visits Alice in prison, awaiting trial for killing her old flame and Bletchley colleague John Richards. Jean is convinced of Alice's innocence and persuades her friends to investigate. They discover that John sent flowers to a young girl, Lizzie Lancaster and a search of her flat reveals she is in possession of a classified military file and a press cutting of an incident where military personnel were injured by a chemical experiment on Salisbury Plain. The women save Lizzie from a hit and run and Jean correctly assumes that she is Alice's daughter. The Bletchley circle believe both mother and daughter have been set up by John's killer who was also responsible for the Salisbury Plain incident.

Alice is to hang, though she tells her friends she assumed Lizzie had killed John and so taken the blame. The women though know that John was killed and the attempt made on Lizzie's life to prevent them revealing what happened on Salisbury Plain and feel that they too are under surveillance. Discovering that the injured soldiers - all clearly guinea pigs as none had families and treated at the same military hospital as Timothy had been, a reluctant Susan visits, claiming she needs his medical records for his overseas posting. She locates a horribly injured young soldier who confirms that he and other victims were deliberately chosen for the heartless experiment but Susan is removed by the military and forced to explain to her husband her past at Bletchley and her current mission. Meanwhile Millie, Jean and Lucy learn that John was employed at Porton Down, a government establishment testing chemical weaponry run by Professor Masters, whom they visit. He is a deranged patriot who believed John and Lizzie were Russian spies and the trio are in danger from him unless Susan can prevail upon the honest colonel at the hospital to help them and save Alice.

Susan and her family have moved to India for her husband's posting whilst Alice, though acquitted of murder, finds work impossible to obtain. Also unemployed Millie runs a nice line in selling black market goods but is abducted and her flat ransacked - though she has hidden wads of her cash in her cooker. Alice, Jean and Lucy look for Millie's business associate Jasper but he has vanished and they surmise that he has fled after cheating his bosses. The women learn that these are the Maltese Magros family, headed by the formidable Marta and they are looking for money appropriated by Jasper. Marta lets Millie go on condition she turns a hefty profit for her but the other women want Millie to turn her back on the whole affair. Millie however is concerned that the Magros are bringing in young Czech girls as sex slaves and wants to stop them. Then they find that Jasper has been murdered.

The women report Jasper's murder but next day Ben, Lucy's colleague at the police station, informs them that the body has disappeared. Realising that there is police corruption at work the women decide they must stop the gang themselves. Jean poses as a potential client and visits Marta at the restaurant she runs, accompanied by Lucy, who uses her photographic memory to memorise the contents of Marta's ledger. It is in code but with Lizzie now at teacher training college next to the old Bletchley site the friends return there and find an old enigma machine. With Alice's expert knowledge they get it working and crack the code, which informs them of the times and locations of Marta's incoming shipments. They tip off customs to intercept the next consignment of sex slaves but realise that Marta is on to them and must act quickly to thwart her.