How Many Episodes Of "The Bletchley Circle" Have You Seen?

During World War Two Susan, a pattern recognition expert, mathematician Millie, information gatherer Jean and Lucy, a young girl with an incredible memory, work as code-breakers at Bletchley Park. In 1952, Susan, now married, hears on the radio about a serial killer and contacts Deputy Chief Commissioner Wainwright, a wartime friend of her husband Timothy, with her theory that there will be another victim in a particular place. However the police find nothing.Susan then decides to call on her former colleagues and, under the guise of a literary club, asks them to join her in the murder investigation. They are initially reluctant until Susan proves that the killer is using a railway timetable to select his victims by plotting their journeys. By following the killer's pattern and cross-referencing information they deduce that the next victim will be Mary Lawrence and she will be murdered at Upton Park. They hurry to their destination but find they are too late.

Susan speaks to a man who claims he is the suspect's psychiatrist and to whom she unwittingly gives her reasons for seeing him. She is lucky to escape and soon realises she has met the murderer but when the police are called to the house he has vanished. She visits Cavendish for more information, only to find that he too has been killed but when she takes her findings to Wainwright he is again dismissive. Research leads the women to find that the serial killer's first victim was Julie Oakwood during the war, and from the leads this gives, Susan makes a final journey to unmask the culprit.

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.

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.