Siblings Jerry and Joanna Burton, from London, take up residence in a house owned by Miss Emily Barton near the quiet town of Lymstock for the last phase of Jerry's recovery from injuries he suffered as a pilot in a
crash landing. Shortly after moving in, they receive an anonymous letter which makes the false accusation that the pair are lovers, not siblings. The Burtons learn that such poison pen letters have been received by many in the town. Despite the fact that many of the accusations are obviously false, recipients are upset by them and fear something worse may happen. Mrs Symmington, the local solicitor's wife, is found dead after receiving a letter stating that her husband Richard was not the father of her second son. Her body is discovered with the letter, a glass containing potassium cyanide, and a torn scrap of paper that reads, "I can't go on". An inquest rules that Mrs Symmington's death was suicide. The local police begin a hunt for the anonymous letter writer. A
Scotland Yard inspector, brought in to help with the investigation, concludes that the writer must be a middle-aged woman among the prominent citizens of Lymstock. Mrs Symmington's daughter by a previous marriage, Megan Hunter, an awkward, frumpy 20-year-old, stays with the Burtons for a few days after losing her mother. The Burtons' housekeeper, Partridge, receives a call from Agnes, the Symmingtons' maidservant, who is distraught and seeks advice. Agnes fails to arrive for their planned meeting; nor is she found at the Symmingtons' when Jerry calls in the evening to check on her. The following day, her body is discovered by Megan in the under-stairs cupboard at the Symmington house. Progress in the murder investigation is slow until the vicar's wife, Mrs Dane Calthrop, invites
Miss Marple to investigate. Jerry expounds to her his ideas on why Agnes was killed. Meanwhile, Elsie Holland, governess to the Symmington's two boys, also receives an anonymous letter. The police observe Aimée Griffith, sister of the local doctor Owen Griffith, typing an address on the same typewriter that was used for the poison pen letters, and they arrest her. Heading to London to see his doctor, Jerry impulsively takes Megan along with him and takes her to Joanna's dressmaker for a complete makeover. Jerry realises he has fallen in love, and when they return to Lymstock, he asks Megan to marry him; she turns him down. He asks Mr Symmington for his permission to court Megan. Miss Marple advises Jerry to leave Megan alone for a day, as she has a task for her. Megan blackmails her stepfather later that evening, implying that she has proof that he killed Mrs Symmington. Mr Symmington coolly pays her an initial instalment of money, while not admitting his guilt. Later in the night, after giving Megan a sleeping drug, he attempts to murder her by putting her head in the gas oven. Jerry and the police are lying in wait. Jerry rescues Megan, and Symmington confesses. The police arrest him for murdering his wife and Agnes. Miss Marple, knowing human nature, reveals that she knew all along that the letters were a diversion, and were not written by a local woman, because none contained true accusations – something locals would be sure to gossip about. Only one person benefited from Mrs Symmington's death: her husband. He is in love with the beautiful Elsie Holland. Planning his wife's murder, he modelled the letters on those in a past case known to him from his legal practice. The police theory about who wrote them was completely wrong. The one letter that Symmington did not write was the one to Elsie; Aimée Griffith, who had been in love with Symmington for years, wrote that. Knowing it would be hard to prove Symmington's guilt, Miss Marple devised the scheme to expose him, enlisting Megan to provoke him to attempt to kill her. Following the successful conclusion of the investigation, Megan realises that she does love Jerry. Jerry buys Miss Barton's house for them. His sister Joanna marries Owen Griffith, and they also remain in Lymstock. Meanwhile, Emily Barton and Aimée Griffith go on a cruise together. ==Principal characters==