Sixteen years after Caroline Crale was convicted for the murder of her husband Amyas, her twenty-one-year-old daughter Carla Lemarchant asks Hercule Poirot to reinvestigate the case, after she was given a letter from her late mother claiming she was innocent, which Carla believes to be true. Poirot agrees to her request and establishes that on the day of the murder at the Crales' home there were five other people present, whom Poirot dubs "the five little pigs" – stockbroker Philip Blake, amateur chemist Meredith Blake (Philip's brother), Caroline's young half-sister Angela Warren, Angela's governess Cecilia Williams and Amyas's painting model Elsa Greer. The police found that Amyas was poisoned by coniine, found in a glass he had drunk from. Caroline confessed to stealing the poison from Meredith's lab, intending to use it to commit suicide. She had brought a cold bottle of beer to Amyas and the police think she had poisoned it, because Amyas was having an affair with his model Elsa.

Poirot interviews the five other suspects and notes that none has an obvious motive. Caroline's half-sister Angela is the only one who believes Caroline was innocent. He assembles them, along with Carla and her fiancé and reveals that Caroline was innocent but chose not to defend herself because she believed Angela had committed the murder. Although Angela had handled the beer bottle she had added nothing to it before her sister took it to Amyas. Caroline later assumed that her sister had added something to the beer as a prank, causing Amyas's death. When they were much younger, Caroline had thrown a paperweight at Angela injuring her face and saw this as a way to atone for that incident.

Poirot gathers the five suspects together and states that the murderer was Elsa Greer. She had taken Amyas' promise to marry her seriously, unaware that he only wanted her to continue as his model until the painting was done. She overheard Amyas reassure his wife that he was not leaving her, felt betrayed and wanted revenge. She had seen Caroline take the poison from Meredith's lab, so she took it from Caroline's room and put it in a glass of warm beer that she gave Amyas. When Caroline later brought him a cold bottle of beer, he commented that "everything tastes foul today", and drank the cold beer from the bottle. This indicated to Poirot that the poison had been in the glass Elsa had given him. Poirot's explanation solves the case to the satisfaction of Carla and her fiancé. Although the chances of getting a pardon for Caroline or a conviction of Elsa are slim with circumstantial evidence, Poirot plans to present his findings to the police. As Elsa leaves, she claims that she is the true victim of the case because she is the one who must live with the guilt.