The Murder on the Links

Hercule Poirot and Captain Hastings travel to Merlinville-sur-Mer, France, to meet Paul Renauld, who has requested their help. When they arrive, local police greet them with the news that Renauld was found dead that morning, stabbed in the back with a knife and left in a newly dug grave adjacent to a local golf course. His wife, Eloise Renauld, claims masked men broke into the villa at 2 am, tied her up, and took her husband away with them. Upon inspecting his body, Eloise collapses with grief at seeing her dead husband. Monsieur Giraud of the Sûreté leads the police investigation, and resents Poirot's involvement.

Poirot notes four key facts about the case: a piece of lead piping is found near the body; only three female servants were in the villa as both Renauld's son Jack and his chauffeur had been sent away; an unknown person visited the day before; and Renauld's immediate neighbour, Madame Daubreuil, had placed 200,000 francs into her bank account over recent weeks.

When Renauld's secretary, Gabriel Stonor, returns from England, he suggests blackmail, as his employer's past is a complete mystery prior to his career in South America. Meanwhile, Hastings unexpectedly encounters a young woman he had met on the train, known only as "Cinderella." She asks to see the crime scene and then disappears with the murder weapon.

Poirot discovers that the case is nearly identical to one from 22 years ago, in which a man called Georges Conneau and his lover, Madame Beroldy, conspired to kill Madame Beroldy's husband. Poirot travels to Paris to discover more about the Conneau murder. He finds a photograph of Madame Beroldy that shows that she is really Madame Daubreuil. On returning, Poirot learns that the body of a tramp has been found, stabbed through the heart with the murder weapon. An examination shows that he died before Renauld's murder from an epileptic seizure and was stabbed later.

Giraud arrests Jack on the basis that he wanted his father's money. Poirot reveals Renauld changed his will two weeks before his murder, disinheriting Jack. Jack is released from prison after Bella Duveen, an English stage performer he loves, confesses to the murder. Both had come across the body on the night of the murder, and each assumed the other had killed Renauld. Poirot reveals neither did, as the real killer was Marthe Daubreuil, daughter of the neighbor.

Poirot elaborates on his theory: Paul Renauld was really Georges Conneau, who returned to France after fleeing years ago. By misfortune, he found that his immediate neighbour would be Madame Beroldy, his old lover; like him, she had changed her identity, becoming Mme. Daubreuil. Blackmailed by her over his past, Renauld found his situation worsened when Jack becomes attracted to her daughter. When a tramp died on his grounds, he saw an opportunity to stage his own death and escape Mme Daubreuil. He would disfigure the tramp's face with the pipe, and then bury the tramp and the pipe beside the golf course, before fleeing the area by train. Anyone who would recognise that the body was not his would be sent away. However, the plan was discovered by Marthe, who followed Renauld and stabbed him after he dug the grave for the tramp's body. Her motive is money; Jack will inherit his father's fortune on his mother's death. To expose Marthe as the killer, Poirot asked Eloise to openly state she will disinherit Jack. Marthe attempts to kill Eloise in her villa but dies in a struggle with Hastings's Cinderella.

Marthe's mother disappears again. Jack and his mother plan to go to South America, joined by Hastings and his Cinderella, who is revealed as Bella Duveen's twin sister Dulcie.