In December 1935, Hercule Poirot is traveling aboard the Orient Express, encountering his friend Signor Bianchi, a director of the company which owns the line. The other passengers traveling in Poirot and Bianchi’s coach are: American widow Harriet Hubbard; American businessman Samuel Ratchett, with his English manservant Edward Beddoes and secretary/translator Hector McQueen; elderly Russian Princess Natalia Dragomiroff and her German maid Hildegarde Schmidt; Hungarian diplomat Count Rudolf Andrenyi and his wife Elena; British Indian Army officer Colonel John Arbuthnot; Mary Debenham, a teacher; Greta Ohlsson, a Swedish missionary; Italian-American car salesman Antonio Foscarelli; and Cyrus Hardman, an American theatrical agent.
The morning after the train’s departure from Istanbul, Ratchett tries to secure Poirot’s services as a bodyguard for $15,000 (equivalent to $286,679 in 2019), as he has received death threats, but Poirot has no interest. That night, Bianchi lets Poirot use his compartment as he goes to sleep in another coach. During the journey, the train is stopped in a snowdrift that has formed on the tracks in Yugoslavia.
The next morning, Ratchett is found stabbed to death in his cabin. Bianchi entreats Poirot to solve the case. They enlist the help of Stavros Constantine, a Greek medical doctor who was travelling in the other coach.
Dr. Constantine finds Ratchett was stabbed 12 times, though some wounds were slight. Poirot’s reconstructed timeline of passenger activities the night before indicate that Ratchett was murdered at about 1:15 a.m. The doors to the other cars were locked, so the murderer is likely one of the passengers in Poirot’s coach, or its French conductor, Pierre Michel.
Found at the crime scene is a fragment of a burned letter. Examining the letter, Poirot discovers that Ratchett was actually Lanfranco Cassetti, a gangster who five years earlier planned the kidnapping and murder of Daisy Armstrong, infant daughter of British Army Colonel Hamish Armstrong and his American wife Sonia. Overcome with grief, the pregnant Mrs. Armstrong went into premature labour and died giving birth to a stillborn baby. A French maidservant named Paulette, suspected of complicity in the kidnapping, committed suicide, only to be found innocent later. Colonel Armstrong, consumed by these tragedies, committed suicide. Cassetti betrayed his partner by fleeing the country with the ransom, and was only revealed to be involved on the eve of his partner’s execution.
Several clues are quickly discovered suggesting that an assassin boarded the train during the night while it was stuck in the snowdrift, murdered Cassetti and then left the train. Mrs. Hubbard reports that she detected the presence of a man in her bed. Then, a button from a sleeping car conductor’s uniform is discovered. Pierre confirms that he is not missing any buttons from his uniform. Later, an entire conductor’s uniform is discovered that does not fit Pierre and is missing a button. In the uniform’s pocket is a conductor’s pass key. Finally, Mrs. Hubbard discovers a bloody dagger. Dr. Constantine confirms that it could have inflicted all the wounds found on Cassetti’s body. Poirot later agrees with Foscarelli that the assassin was likely a member of a rival Mafia gang, exacting vengeance as part of a Mafia feud.
As these clues are being discovered, Poirot begins interviewing the passengers. He learns that: McQueen was the son of the District Attorney who prosecuted the case and was very fond of Mrs. Armstrong; Beddoes had been a British Army batman; Countess Andrenyi is of German descent and her maiden name is Grunwald; Greta Ohlsson has a limited knowledge of the English language and has been to America; Pierre Michel’s daughter died five years earlier of scarlet fever; Col. Arbuthnot, who displays a knowledge of Armstrong’s military decorations, plans to wed Mary Debenham. When Poirot interviews Princess Dragomiroff he discovers she is a great friend of Linda Arden, Mrs. Armstrong’s mother; the Princess was Sonia’s godmother. He learns that the Armstrongs had a butler, a secretary, a cook, a chauffeur and a nursemaid to Daisy. Poirot flatters Hildegarde Schmidt by saying he knows a good cook when he sees one, and she responds to his request for a photo of the maid Paulette, with whom Miss Schmidt was friendly. Foscarelli vehemently denies ever having been in private service as a chauffeur. Hardman reveals he is, in fact, a Pinkerton detective hired to guard Cassetti. When shown the photo of Paulette, Hardman breaks down and reveals he knew her.
Poirot gathers the suspects together, stating he has formulated two possible scenarios. The first, his simple solution, is the one suggesting that Cassetti’s murder was the result of a Mafia feud. The second, his complex solution, links all the suspects on the coach to the Armstrong case. In addition to the earlier self-incriminating revelations by Hardman, McQueen, Miss Schmidt and the Princess, the Princess had incriminated three others: when asked Mrs Armstrong’s maiden name, she replied “Greenwood”, the English for “Grunwald”, allowing Poirot to deduce that Countess Elena was Mrs Armstrong’s sister, and Count Andrenyi her brother-in-law; the Princess also claimed the secretary’s name was “Miss Freebody”, so Poirot deduced the secretary was in fact Mary Debenham (as in the London department store Debenham and Freebody). Poirot then presents the motives of the other suspects: Pierre was Paulette’s father; Beddoes was Colonel Armstrong’s army batman and the family butler; Miss Ohlsson was Daisy’s nursemaid (having inadvertently shown an understanding of complex English words); Col. Arbuthnot was a close army friend of Armstrong’s; Foscarelli was the Armstrongs’ private chauffeur; Hardman was a police officer in love with Paulette and Mrs Hubbard is Linda Arden, Mrs Armstrong’s mother. McQueen had drugged Cassetti so each of the passengers could stab him – with the Andrenyis stabbing Cassetti together, totaling 12 stab wounds. The noises which disturbed Poirot’s sleep were contrived to obfuscate the time of death.
Poirot asks Bianchi to choose one of the two solutions to present to the police once the train is freed from the snowdrift. He admits that the Yugoslavian police will likely prefer the simple solution. Bianchi decides that Cassetti deserved to die, and elects the simple solution. Poirot agrees, admitting he will struggle with his conscience when he gives his report to the police. The passengers celebrate with champagne as the train is freed of the snow and resumes its journey.