When nobody will believe she witnessed a murder, elderly sleuth Miss Marple takes a job as a maid to ferret out clues.

While reading a mystery novel on a train from London, the elderly Miss Marple sees a woman being strangled by a man on a passing train. The police, however, are unable to find any trace of the body and dismiss her story as the imaginings of a doddering old woman. Highly indignant, she decides to investigate for herself. Believing the body was thrown from the passing train near the Ackenthorpe estate, she takes a job there as a maid; and her suspicions prove correct when her snooping uncovers the woman's body in a deserted outbuilding. At first the victim is believed to be a French girl once married to an Ackenthorpe killed in World War II. Consequently, suspicion naturally falls on the elderly Ackenthorpe's four remaining children, each of whom might resort to murder to prevent sharing their inheritance with another heir. Two more sons of the aged patriarch are murdered, and Miss Marple deduces the identity of the murderer and sets a trap. Her plan exposes the family physician, Quimper, as the killer. (Quimper murdered his own wife on the train and made it appear that she was the French-born wife. He then planned to kill off the sons before marrying Emma Ackenthorpe, the only daughter, who would eventually inherit the entire fortune.)