Hercule Poirot and Captain Hastings travel to Merlinville-sur-Mer, France, to meet Paul Renauld, who has requested their help. Upon arriving at his home, the Villa Genevieve, local police greet them with news that he has been found dead that morning. Renauld had been stabbed in the back with a letter opener 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 S ret leads the police investigation, and resents Poirot's involvement; Monsieur Hautet, the Examining Magistrate, is more open to sharing key information with him.
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, whom Renauld urged to leave immediately; 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 met before, known to him as "Cinderella", who asks to see the crime scene, and then mysteriously disappears with the murder weapon. Poirot later travels to Paris to research the case's similarities to that of a murder case from 22 years ago, which has only one difference - the killer, Georges Conneau, later confessed to the crime, in which he and his lover, Madame Beroldy, had plotted to kill her husband and claim that the murder was carried out by masked intruders; both disappeared soon afterwards.