In post-World War II Venice, retired detective Poirot attends a seance where a murder occurs. He must uncover the killer's identity in a palazzo filled with suspects.

While on vacation on the Nile, Hercule Poirot investigates the murder of a young heiress. In World War I, Poirot devises a strategy to advance his Belgian company but gets injured. In 1937, at a London club, Poirot watches a performance and sees Linnet Ridgeway dancing passionately with her fiance Simon Doyle. After six weeks, in Egypt, Poirot encounters his friend Bouc and joins them at the hotel to celebrate Linnet and Simon's wedding. Others join their honeymoon trip on the cruise ship S.S. Karnak. Linnet asks Poirot for protection from the obsessive Jackie, who has stalked them to Egypt. To escape Jackie, the group boards the Karnak. Poirot interrogates the guests and discovers their various motives to harm Linnet. Poirot investigates further, finding clues and uncovering the truth behind the murders. As the passengers disembark, Poirot is unable to voice his feelings to Salome. Six months later, he visits her club to watch her rehearse, alone in the dark.

When a murder occurs on a train on which he's travelling, celebrated detective Hercule Poirot is recruited to solve the case. In 1934, famous detective Hercule Poirot solves a theft at the Church of the Holy Sepulchre in Jerusalem. The obsessive compulsive Belgian-who seeks balance in life, and considers his case-solving ability to see a lie amid truth a curse-wants to rest in Istanbul but must return to London for another case. His friend Bouc, director of the Orient Express, offers him a room on the train. Poirot refuses businessman Samuel Ratchett's offer to hire him as his bodyguard during the three-day journey, as Ratchett has received threatening letters from an unknown party. That night Poirot hears strange noises coming from Ratchett's compartment, and sees someone in a red kimono running down the hallway. An avalanche derails the train and strands the passengers. The next morning Poirot learns Ratchett was murdered during the night after being stabbed a dozen times. Poirot and Bouc investigate passengers as repairs begin. Evidence indicates that Ratchett was murdered by one person, with Caroline Hubbard claiming that a man was in her compartment during the night and that since her cabin was next to Ratchett, the murderer would have entered Ratchett's cabin through hers. Poirot discovers a partially destroyed note connecting Ratchett to the kidnapping of Daisy Armstrong, a child who was abducted from her bedroom and held for ransom. After the ransom was paid, Daisy was found murdered. Ratchett is identified as John Cassetti, Daisy's kidnapper and murderer. Her death caused her mother Sonia to die after giving premature birth to a stillborn baby; her father, Colonel Armstrong, then committed suicide. The family's nursemaid Susanne was arrested and hanged herself while in police custody, but exonerated after her death. The train compartments were locked at night, so the only suspects were the people in that particular compartment. Also that compartment had a guard who said he saw nobody enter Ratchett's cabin during the time of his death. Ratchett's coffee is found to be drugged, but his Valet Henry does not know how the coffee was drugged before or after he delivered it to Ratchett. Then Poirot finds Hector trying to burn the ledgers of Ratchett's accounts. Poirot stops him and accuses him of stealing from Ratchett and hence killing him for the fear of being found out. But Dr Artbutnot confirms that Hector was drinking with him all night. More evidence is found, including a bloodstained handkerchief and the button of a conductor's uniform found in Hubbard's compartment. The uniform is later found, as is the kimono, discovered in Poirot's own suitcase. Hubbard is stabbed in the back; she survives but cannot identify the culprit. Poirot discovers several of the passengers have direct connections to the Armstrong family. While interviewing governess Mary Debenham, John Arbuthnot shoots Poirot in the shoulder claiming that he is the murderer, but is stopped by Bouc. Poirot realizes that Arbuthnot never meant to kill him. Poirot then interviews the count and countess Andrenyi. He finds that the countess's passport has been tampered and deduces that her first name is not Elena, but Helena. He then deduces that Helena is in fact the sister of Sonia Armstrong, who is living in permanent fear from the events of the past. Poirot meets the suspects outside the train, offering two theories of how Ratchett died. The first is simple: A murderer disguised as a conductor boarded the train, murdered Ratchett, and fled. The second is more complex: Every suspect is connected to the Armstrongs, Susanne, or her trial in some way-including those who had pretended otherwise-and had motive to kill Ratchett/Cassetti. Hubbard is revealed to be Linda Arden, former stage actress and Sonia Armstrong's mother. Poirot reveals that the 13 other passengers on the train, and the train conductor, were all connected to the Armstrong family in some way. Hubbard admits that she gathered everyone and planned the murder. All of the suspects took turns stabbing Ratchett. Arbuthnot stabbed Hubbard without endangering her life to convince Poirot of a lone killer. Poirot challenges the passengers to shoot him with his gun since he is the only one who can expose their plot; Bouc can lie but Poirot-obsessed with truth and balance-cannot. Hubbard grabs the gun and tries to kill herself. The gun is unloaded; Poirot wanted to see how the suspects would react. As the train is put back on track Poirot concludes that justice is impossible in the case, as Ratchett deserved death; for the first time he will have to live with a lie and imbalance. Poirot presents the lone killer theory to the Yugoslavian police, allowing the suspects to leave. As he disembarks the train a messenger asks him to investigate a death on the Nile. Poirot takes the case.