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.
Episode 2. Episode 2
(AIR DATE 04/28/2013)
The women collate information about the schedules of trains the victims had been on and use this to identify potential suspects. Susan gives the police names of three potential perpetrators. As Susan gives the police the information, Jean and Lucy discover seven similar murders that the police think they have solved, but where the women believe innocent men have been framed and convicted. The police arrest one of the men who was among the names given to them by Susan after finding evidence connecting him to the victims. The women devise a plan to trap the killer using Lucy as bait but it backfires when she goes with the wrong man. Another suspect emerges from a top secret war department headed by Cavendish and Susan comes face to face with the killer at a closed mental hospital.
Episode 3. Episode 3
(AIR DATE 05/05/2013)
Susan returns with the police but the killer has gone. Susan finds a coded message in her home with Cavendish's address, and going there finds him dead. A postcard on Cavendish's desk provides a clue; and Susan, following the thread, walks alone into a trap set for her by the killer.