Hercule Poirot is a neat, egg-headed retired Belgian detective who lives in London with Captain Hastings. He relies on psychological reasoning and his "little grey cells" rather than physical clue-hunting. When he receives an urgent plea for help from Paul Renauld, he travels to France only to find his new client murdered.

Captain Hastings is a former British army officer who shares London rooms with Poirot and works as a private secretary. He possesses a highly romantic imagination and frequently judges situations based on physical appearances. He falls quickly for an American acrobat on a train, an attachment that tests his loyalty to Poirot's investigation.

Paul Renauld is a wealthy French-Canadian millionaire who built a fortune in South America before settling at the Villa Geneviève in Merlinville-sur-Mer. Shortly before his death, he sends a desperate letter to Poirot claiming his life is in danger and alters his will to disinherit his son. He maintains strict privacy about his past and pays massive sums of money to his neighbor.

Eloise is the tall, silver-haired wife of Paul Renauld. She provides a detailed account of two masked men breaking into their home and abducting her husband to extract a secret. Her genuine grief over her husband's death complicates the initial theories of the investigating detectives.

Jack is the passionate son of Paul and Eloise Renauld. He recently had a fierce argument with his father over his desire to marry Marthe Daubreuil. His father ordered him to travel to South America right before the murder, but Jack secretly returned to Merlinville on the night of the crime, making him a prime suspect.

Marthe is the beautiful, anxious daughter of the mysterious Madame Daubreuil. She is engaged in a secret romance with Jack Renauld, a match strictly opposed by Jack's parents. She watches the police investigation with intense worry, particularly regarding Jack's status as a suspect.

Madame Daubreuil is the enigmatic neighbor of the Renauld family who lives quietly but receives massive cash deposits from Paul Renauld. She refuses to answer official questions about her past or her relationship with the dead man. Poirot recognizes her face from a decades-old murder case in Belgium.

Giraud is an arrogant, physically energetic detective from the Paris Sûreté. He represents the modern school of forensic investigation, focusing intently on footprints, matchsticks, and physical traces left at the scene. He views Poirot's methods as outdated and constantly challenges the Belgian detective's authority.

Gabriel Stonor serves as Paul Renauld's practical, level-headed English secretary. He handles Renauld's finances and confirms that the large payments to Madame Daubreuil were likely blackmail rather than gifts of infatuation. He remains steadfast in his belief that the Renaulds were a devoted couple.

Dulcie is an American-born acrobat and variety show performer who meets Hastings on a train and introduces herself as Cinderella. She shows intense curiosity about the murder at the Villa Geneviève and uses her charm to convince Hastings to show her the crime scene.

Bella is a stage performer and Dulcie's twin sister. She shares a complicated romantic history with Jack Renauld, leading her to write aggressive letters declaring she would rather kill him than lose him to another woman.

Lucien Bex is the Commissary of Police in Merlinville and an old acquaintance of Poirot. He provides the initial briefing on the murder and assists the examining magistrate in questioning the household staff.

Hautet is the examining magistrate overseeing the legal proceedings of the Renauld murder. He conducts the formal interrogations of the family and neighbors, trying to piece together the events of the night.

Françoise is the housekeeper at the Villa Geneviève. She testifies that Madame Daubreuil regularly visited Paul Renauld and suggests a strained atmosphere existed in the house shortly before the murder.

Denise is a maid at the villa who insists that Paul Renauld's late-night visitor was a young Englishwoman, directly contradicting the housekeeper's testimony about Madame Daubreuil.

Léonie is the maid who discovers Eloise Renauld bound and gagged in her bedroom on the morning of the murder, raising the first alarm of the tragedy.

Auguste is the gardener at the Villa Geneviève. Poirot asks him to step into a flower bed to demonstrate how his boot prints match the mysterious tracks found at the scene.

Dr. Durand is the local physician who determines the time of death for the victims found on the property, providing crucial medical testimony that shapes the detectives' timelines.