The Mystery of the Blue Train recap

Rufus Van Aldin gives his estranged daughter Ruth Kettering the Heart of Fire, a famous ruby, before she takes the Blue Train to the Riviera. Ruth plans to meet her lover, the Comte de la Roche, while her impecunious husband Derek and the dancer Mirelle complicate her private life. On the journey Ruth befriends Katherine Grey, newly wealthy after years as a companion. Ruth is found murdered in her compartment and the ruby is gone. Poirot investigates at Van Aldin's request, testing the evidence of Ruth's maid Ada Mason, the count's schemes, Derek's movements, and Mirelle's jealousy. He discovers that Van Aldin's trusted secretary, Major Richard Knighton, is the international thief known as the Marquis. Knighton killed Ruth for the ruby, aided by Mason, whose story about a man in the compartment was manufactured to misdirect the inquiry. Knighton is exposed, Derek is cleared, and Katherine emerges from the case with the prospect of a life with Derek rather than the cautious isolation she expected.

American financier Rufus Van Aldin purchases the Heart of Fire, a celebrated ruby, and gives it to his daughter Ruth Kettering. Ruth is separated from her charming but penniless husband Derek, and Van Aldin urges her to complete the divorce. He is equally alarmed by her renewed connection with the Comte de la Roche, a polished adventurer who knows about the jewel. Van Aldin's trusted secretary, Major Richard Knighton, manages the arrangements. Elsewhere, Katherine Grey finishes years of service as companion to a difficult old woman and unexpectedly inherits a fortune. She decides to travel, entering a world in which her good sense is more reliable than her new money.

Katherine boards the Blue Train for the Riviera and falls into conversation with Ruth, who hints at an unhappy marriage and an important decision. In Paris Ruth dismisses her maid, Ada Mason, for the night. The next day Ruth is found dead in her compartment, her face badly injured and the Heart of Fire missing. Mason says a man was with Ruth in Paris but that she did not see him clearly. The known appointment with the Comte de la Roche immediately directs suspicion toward him. Derek Kettering has also been travelling in the region, and Ruth's death gives him a financial interest as her husband. Poirot, another passenger, agrees to help Van Aldin while the French authorities begin their inquiry.

At the Riviera Poirot compares the accounts of the train journey with the relationships surrounding Ruth. Katherine stays with her cousin Lady Rosalie Tamplin and Rosalie's sharp-eyed daughter Lenox, placing her near Derek and his lover Mirelle. Derek admits enough to look reckless but denies killing Ruth; Mirelle is jealous of the ease between him and Katherine. The count's interest in Ruth's jewels is real, yet Poirot doubts that his familiar form of fraud explains every feature of the murder. Mason's evidence remains central because it supplies the unknown man in Ruth's compartment, but her inability to identify him makes the story unusually convenient. Knighton assists Van Aldin and grows close to Katherine while Poirot quietly checks what each person knew about the ruby.

Poirot follows the jewel rather than the most visible romance. In Paris he consults Demetrius Papopolous, an old contact with detailed knowledge of stolen gems, and learns more about the international thief called the Marquis. The Heart of Fire is valuable enough to draw a professional scheme, and the mutilation of Ruth's face may have been meant to confuse identification as well as to show violence. The count can be tied to an attempt to profit from Ruth, but evidence against him does not account cleanly for the timing aboard the train. Poirot also studies Mason's prepared recollection, Derek's movements, Mirelle's changing statements, and the access enjoyed by people in Van Aldin's confidence. Katherine remains important because Ruth spoke to her without knowing her and because Katherine notices character without trying to build a case.

The inquiry narrows as Poirot separates several genuine deceptions from the murder itself. The Comte de la Roche has lied and schemed for the ruby, but his dishonesty is too obvious to settle the case. Mirelle tries to use what she knows of Derek's movements to keep control of him, while Derek's feelings for Katherine become harder to disguise. Poirot returns repeatedly to the information available before the journey: who knew Van Aldin had bought the Heart of Fire, who knew Ruth would carry it, and who could arrange testimony inside the sleeping car. Papopolous's information about the Marquis suggests that the robbery was not an impulsive act. Poirot applies pressure to the witness whose account created the apparent stranger in Ruth's compartment and prepares to confront the person behind it.

Poirot exposes Major Richard Knighton as Ruth Kettering's murderer and as the master jewel thief known as the Marquis. Knighton's place beside Van Aldin gave him advance knowledge of the Heart of Fire and Ruth's journey. Ada Mason was his accomplice, and her report of an indistinct man in Ruth's compartment was constructed to direct attention toward the Comte de la Roche while remaining flexible enough to survive questioning. Knighton killed Ruth aboard the train and stole the ruby, using the count's plans, Derek's financial interest, and Mirelle's jealousy as layers of misdirection. Poirot's inquiries through Papopolous connect the professional theft to the supposedly respectable secretary, and Mason's story fails under pressure. The ruby is recovered, Derek is cleared, and Knighton's interest in Katherine is stripped of its reassuring appearance. Katherine comes through the case independently wealthy and emotionally changed, with Derek now free and the possibility of a future between them.

Poirot identifies Major Richard Knighton as both Ruth Kettering's murderer and the elusive jewel thief known as the Marquis. Knighton's trusted position gave him full knowledge of Van Aldin's gift and Ruth's travel plans. Ada Mason was his accomplice: her account of seeing a man in Ruth's compartment was designed to point toward the Comte de la Roche, and her description could be adjusted as the investigation required. Knighton killed Ruth on the train and took the Heart of Fire, while the count's planned meeting and Derek's presence supplied convenient alternative suspects. Poirot uses Papopolous's knowledge of the jewel trade, Knighton's handling of evidence, and pressure on Mason to break the prepared story. The ruby is recovered and the count's fraud exposed, though he is not the killer. Derek is cleared of Ruth's murder. Mirelle's attempt to hold him with incriminating claims fails, and his attachment to Katherine becomes plain. Knighton's courtship of Katherine is revealed as part concealment and part miscalculation; Katherine survives the case and is free to choose a future with Derek.