Superintendent Spence informs Belgian detective Hercule Poirot of the case of Mrs. McGinty, an elderly lady who was apparently killed by her lodger, James Bentley, for thirty pounds that she kept under a floorboard. Bentley is soon to be executed for the crime, but Spence does not think he is guilty. Poirot agrees to go to the town of Broadhinny and investigate the murder further. Poirot finds that Mrs. McGinty often worked at the houses of people in the village. No one wants to talk to Poirot, and most agree Bentley is the killer. Eventually, Poirot finds a newspaper article which discusses to current whereabouts of people connected with famous murder cases, that also includes photographs of them. Poirot concludes Mrs. McGinty recognized one of the photographs in someone's house. Someone then killed her to keep their identity concealed. Poirot and Spence, using the ages of people in the town, conclude that someone is either Lily Gamboll, who committed murder with a meat cleaver as a child, or Eva Kane, who had been the love interest who inspired a man to murder his wife and bury her in the cellar. Another possibility is that someone is Evelyn Hope, the daughter of Eva Kane.
Poirot discovers the murder weapon, a sugar hammer, on open display at his boarding house and accessible to all the suspects. In an attempt to flush out the murderer, Poirot claims to know more than he does, but he is almost pushed under a train. Poirot then decides to show most of the suspects the pictures at a party. Mrs. Upward claims to have seen the picture of Lily Gamboll, but does not say where. During the maid's night off, Mrs. Upward's son Robin Upward, a theatre director and Mrs. Ariadne Oliver, a famed mystery novelist who has been working on a theatre adaption with Robin, leave for an evening at the theatre, leaving Mrs. Upward alone at the house. When they return, they find Mrs. Upward strangled to death. She has evidently had coffee with her murderer, and the evidence of lipstick on a coffee cup and perfume in the air points to a woman committing the crime. Mrs. Upward had invited three people to her house that night: Eve Carpenter, Deirdre Henderson and Shelagh Rendell. Any of the three women could be someone from the photographs. Additionally, the postmistress' assistant, Edna, sees someone with blonde hair enter the house, which points to either Carpenter or Rendell, as Henderson is not blonde. Confusing matters even further is the fact that a book is discovered in Mrs. Upward's house with Evelyn Hope's name it in, which suggests that Mrs. Upward is actually Eva Kane. Poirot connects the final piece of the puzzle when he finds the picture Mrs. McGinty saw at Maureen Summerhayes' house. It is of Eva Kane and has the inscription “my mother” on the back. Now with the complete story, Poirot gathers all the suspects together and reveals to them the murderer: Robin Upward.
Robin is Eva Kane's son, Evelyn; the child was a boy, not a girl. Mrs. Upward had not known who Robin's mother was and he knew that any scandal would be to his detriment. Mrs. McGinty saw the picture while working at the Upward house and assumed the picture was of Mrs. Upward as a young woman. Robin killed her to prevent her from telling anyone who might recognize the picture of Eva Kane. Mrs. Upward thought the picture of Eva Kane to be similar to a picture Robin showed her of his mother, whose back story he made up. She wanted to confront Robin by herself, so she pointed to the wrong picture. Robin, however, sensed the truth and killed her before leaving for the play. Then he planted the evidence and made the calls to make it appear that a woman committed the crime. At this point Robin still had the picture, but rather than destroy it, he kept it and planted it at Mrs. Summerhayes' house in order to incriminate her. But Poirot had gone through the drawer earlier and did not see the picture, so he knew it had been planted. Robin is then taken away and imprisoned.
Further revelations are also made. Eve Carpenter wanted to conceal her past for reasons of her own, which was why she didn't cooperate in the investigation. Poirot discovers that Dr. Rendell may have killed his first wife, which led Mrs. Rendell to talk about anonymous letters she'd received warning her of the fact. Thinking that Poirot was actually in Broadhinny to investigate the potential murder of Dr. Rendell's first wife, not the murder of an unimportant charwoman, Poirot suspects it was Dr. Rendell who tried to push him off the platform and under a train. Maude Williams is actually the daughter of Eva Kane's lover. She came to Mrs. Upward's house, thinking Mrs. Upward was Eva Kane, with the intent to kill her, but left once she found her dead. Poirot tells her he will not mention that fact to anyone. Finally, Poirot reveals to Superintendent Spence his plan to pair off Deirdre Henderson with James Bentley.