uring 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.

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