While visiting her American school friend Ruth Van Rydock in London, Miss Marple learns that Ruth is seriously concerned for her sister Carrie Louise. She asks Miss Marple to visit Carrie Louise at Stonygates, her home in England. Miss Marple agrees to the visit. She is impressed by the size of the Victorian mansion, which now has a separate building for delinquent boys, the cause which engages Carrie Louise and her third husband, Lewis Serrocold. Carrie Louise has her family living with her, as her granddaughter Gina has brought her American husband Walter to England to meet her family. Daughter Mildred Strete moved back home after she was widowed. Stepsons Stephen and Alexis Restarick, now grown, are frequent visitors and are present during Miss Marple’s visit. One of the first people Miss Marple encounters is Edgar Lawson, a young man acting as a secretary to Serrocold; Lawson shows clear signs of paranoid schizophrenia, but these are largely ignored.
Miss Marple learns that Carrie Louise has experienced health problems incidental to old age. Nevertheless, Miss Marple is pleased to see that Carrie Louise is still the sweet, idealistic, and loving person she has known.
An unexpected visitor arrives at Stonygates—Christian Gulbrandsen, Carrie Louise’s stepson from her first marriage and a trustee of the charitable foundation that his father set up with the wealth he generated in his life. Lewis Serrocold walks from the train station and meets Christian on the terrace. Miss Marple watches them through her bird-watcher’s binoculars and tries to learn the reason for Christian’s unexpected visit. She hears a few phrases concerning the importance of keeping a problem from Carrie Louise, and that the two men agree to call for outside help. Both men enter for dinner, and afterwards, Christian retires to his room to write letters.
The rest of the household is held entranced by hearing a scene that plays out behind the locked door of Lewis Serrocold’s office. Lawson enters it with a gun and speaks loudly to Lewis, claiming Lewis is his father and has treated him badly. Lawson threatens to shoot him, while Lewis tries to calm the young man.
Tension is added to the scene because an electrical problem has caused most of the Great Hall outside Lewis’s office to go dark. Walter knows how to fix the overloaded fuse, so he leaves the room to fix it and then rejoins the group. While Edgar Lawson is ranting to Lewis, the family hear shots and intervene by trying to open the door. Another shot (not fired in Lewis’s office) had been heard by some, but not all. When the door to Lewis’s office is finally opened, Lewis scoffs at any concern for himself, and they see that the shots Lawson fired had hit the wall. Lawson collapses in tears and apologies.
Meanwhile, “Jolly” Juliet Bellever, housekeeper and companion to Carrie Louise, had gone out to find the key to the office. She returns to the room and says she has called the police, not because of the scene between Edgar Lawson and Lewis Serrocold, but because she has found Christian Gulbrandsen dead in his room from a gunshot.
Lewis proceeds to Christian’s room, followed by Carrie Louise and Miss Marple. Alexis Restarick arrives at the house. His brother Stephen was already there, playing the piano after dinner. Then the police arrive.
Inspector Curry quickly establishes that none of the people from the facility for delinquent boys are involved, nor any of the servants either. Curry discovers that there was a sheet of paper in the typewriter when Jolly entered the dead man’s room. Lewis admits to removing it, explaining that he had feared his wife would read it and discover that the reason for Christian’s visit was his fear that someone had been poisoning Carrie Louise. Lewis suggests that the poison is in her medicine, a liquid which is shown to contain arsenic.
Miss Marple comments that most of the family would be pleased if Walter were found to be the killer, but Christian was not killed by Walter’s gun, which was in Lawson’s hand during the interval. Police find the murder weapon under some music inside the piano bench.
Alexis explains that his drive to the house was slowed by the fog, and that what he saw and heard in the fog, such as a shot and the sound of someone running, gave him an idea for a stage set. Alexis envisions the house as a stage, which causes Miss Marple to begin thinking differently about the murder. The next evening, Alexis and the boy Ernie Gregg are killed by stage weights.
Miss Marple explains to the police how one person could run from Lewis’s study to Christian’s room along the terrace in under two minutes—Lewis Serrocold. Lawson spoke as both himself and Lewis, while Lewis killed Christian and returned out of breath. The suspicion of Carrie Louise’s poisoning was a ruse created by Lewis. The real reason for Christian Gulbrandsen’s visit was that he had learned that Lewis was embezzling from the Gulbrandsen Trust. The reader also learns that Edgar Lawson was only pretending to be schizophrenic; in reality, he is the illegitimate son of Lewis.
When confronted by the police, Lawson flees the house, jumping into an old boat to cross a lake on the property. The boat begins to sink, so Lewis Serrocold jumps into the lake to rescue his son. Both are caught in the reeds lining the lake and drown before the police can reach them. The scene of these deaths ends with Carrie Louise walking indoors with her daughter Mildred, as a new solidarity between mother and daughter is manifest. Carrie Louise’s granddaughter Gina agrees to head back to America with her American husband Walter, averting a threatened separation.