At a train platform in Aleppo, Syria, a detective named Hercule Poirot boards a train to Istanbul. In the train car are two other passengers: Colonel Arbuthnot, a British officer stationed in India, and Mary Debenham, a young English governess. The two seem acquainted from traveling on the same train, and Poirot observes a polite conversation between them that’s peppered with ominous references to a time in the future “When it’s all over.”
At the hotel in Istanbul, Poirot gets a telegram requesting him in London, so he arranges travel by the Orient Express and in the meantime finds dinner at the hotel restaurant. There, he encounters an old friend: M. Bouc, the director of the Wagons-Lit company, which runs the Orient Express. Two Americans, a young man and an older one, are also dining. Poirot remarks on the appearance of the older of the two men; at first glance, he looks harmless, but a closer look reveals a malevolent quality.
Boarding the Orient Express, M. Bouc and Poirot find that all the compartments are taken, which is unusual for this time of year. Poirot is forced to share a second-class cabin with the young American from the hotel restaurant. Later in the dining car, Poirot takes some time to observe the thirteen other passengers in the train car, who are people of “all classes and nationalities.” These include Ms. Debenham and Colonel Arbuthnot from the previous leg of the journey, as well as the two Americans from the hotel restaurant, the younger Hector Macqueen and the older Mr. Ratchett. Poirot also notices an elderly Russian lady, Princess Dragomiroff, and a boorish American woman, Mrs. Hubbard. Also assembled are the Hungarian nobles Countess and Count Andrenyi, a Swedish woman named Greta Ohlsson, and the Italian Antonio Foscarelli.
After dinner, Mr. Ratchett recognizes Poirot as a detective and asks him to take on a job finding the source of some threats that he’s received, but Poirot refuses him. Having traded rooms with M. Bouc, Poirot retires and falls asleep, but he’s awakened some time later by a loud groan from the next cabin, occupied by Mr. Ratchett. The conductor asks at the door, and the occupant responds in French that everything’s okay.
Unable to go to sleep, Poirot talks to the conductor, Pierre Michel, who confides that Mrs. Hubbard, the American woman, claims to have seen a shadowy figure in her room. He also mentions that the train is stopped in Yugoslavia due to heavy snow. Before Poirot falls asleep, he hears a thud and looks into the hallway to see a figure in a scarlet kimono rushing by.
In the morning, M. Bouc calls for Poirot and informs him that Mr. Ratchett has been killed in the night. A Greek doctor named Dr. Constantine examines the body and finds that Ratchett has been stabbed multiple times. As they are snowed-in and have no hope of police assistance, M. Bouc asks Poirot to take on the case, and Poirot agrees.
Poirot notifies Ratchett’s secretary, Mr. Macqueen, and he reveals that Ratchett had received several threatening letters, but generally he knows little about Ratchett’s history. Poirot dismisses him and examines the body and crime scene. Ratchett has been stabbed twelve times, but the wounds appear to have been inflicted by two people of varying physical strength. What’s more, some stab wounds were delivered after Ratchett was already dead. In Ratchett’s room, Poirot finds many clues that appear a bit too convenient, as if this were a “detective’s novel”: a woman’s handkerchief monogrammed with an “H,” a stopped pocket watch purporting to give the time of the murder, a pipe-cleaner, and a burnt letter fragment mentioning the name “Daisy Armstrong.”
Poirot links the name Daisy Armstrong to a criminal case in America. Daisy Armstrong was the daughter of Colonel Armstrong and Sonia Armstrong, a well-respected and connected American couple. When Daisy was three, she was abducted from her home. The Armstrongs paid a huge ransom to the kidnappers, but afterward Daisy was discovered to be already dead. Overcome with grief, Sonia Armstrong died and Colonel Armstrong committed suicide. Daisy’s French nursemaid also killed herself after being falsely targeted for the kidnapping. A gangster named Cassetti was all but convicted of the crime, but he managed to escape justice by bribing high-ranking officials. Poirot concludes that Ratchett is actually Cassetti, having fled to Europe and changed his name.
Poirot then begins interviews with the passengers. Each seems surprised that Mr. Ratchett was actually the criminal Cassetti, and each has an alibi for the time when he was killed. The American Mrs. Hubbard insists that the murderer was in her cabin last night, although she claims the door between her room and Ratchett’s was barred the previous night. Mrs. Hubbard produces a button from a conductor’s uniform that she found near her window and seems mollified when Poirot accepts it as evidence.
Before interviewing the next person on the list, Princess Dragomiroff, Poirot confronts the conductor Pierre Michel with the button Mrs. Hubbard found. Pierre insists that he didn’t lose a button and he calls a conductor from another train car to confirm his alibi. In an interview with Princess Dragomiroff, it’s revealed that she did know the Armstrong family and was, in fact, Sonia Armstrong’s godmother. She vaguely mentions the actress Linda Arden, who was Sonia’s mother, as well as a younger sister of Sonia’s who moved to England.
Poirot discovers a few significant revelations while talking to the passengers. Cyrus Hardman reveals that he’s not a typewriter salesman but a private detective hired by Mr. Ratchett to protect him. Further, Ratchett told him to expect a small dark man with a feminine voice. A few passengers claim to have seen this small dark man in a conductor’s uniform the previous night, as well as a tall woman in a scarlet kimono.
After interviewing all the passengers, Poirot sums up the evidence, much of it contradictory. Poirot proposes to search each passenger’s luggage. Suddenly, Mrs. Hubbard bursts in to say she’s discovered a knife in her bag. Poirot identifies the knife as the murder weapon, but provides few other clues.
They begin searching the luggage and find a conductor uniform in Hildegarde Schmidt’s luggage, which Poirot predicted, but doesn’t mark her as guilty in his eyes. Poirot also has an extended conversation with Ms. Debenham where he confronts her with the conversation he overheard on the train from Syria, but she stubbornly refuses to explain it. Finally, Poirot finds the scarlet kimono in his own luggage, which he understands as a “defiance” from the murderer or murderers.
Poirot makes a list of the evidence and the remaining questions. A pocket watch was found on Ratchett stopped at 1:15, but it’s unclear whether the crime was committed then or the murderer wants them to think it was committed then. They have an expensive handkerchief monogrammed with an “H,” which no passenger will claim. They also have a pipe-cleaner, which seems to point to Colonel Arbuthnot as the only pipe smoker, but he only shrugged when confronted with it. Poirot noticed a grease spot on Countess Andrenyi’s passport near her first name, which suggests an alteration.
Poirot begins to confront passengers with the results of his deductions. The Countess’s first name is not Elena but Helena. The Count had altered her passport after the handkerchief with an “H” was found in Ratchett’s cabin, but the handkerchief isn’t hers, and they profess their innocence. However, Helena does admit that she’s actually Helena Goldenberg, Sonia Armstrong’s sister, which explains her desperation to escape suspicion for Ratchett’s murder. Afterward, Princess Dragomiroff steps forward to claim the handkerchief.
Next, Poirot rounds on Ms. Debenham, who he has identified as Helena's former governess, which she admits, while Colonel Arbuthnot leaps to her defense. Then follows a series of admissions from passengers that have not been forthright: Antonio Foscarelli admits that he was the Armstrong’s chauffer, Greta Ohlsson admits that she was Daisy’s nurse, and Masterman, Mr. Ratchett’s valet, admits he was Colonel Armstrong’s assistant in World War I. This is enough for Poirot to call all the passengers to assemble in the dining car so he can propose the solution to the crime.
Poirot actually has two solutions. The first is that the small, dark man Hardman mentioned boarded the train at a stop in Belgrade or Vincovci, changed into a conductor uniform, stabbed Ratchett twelve times, and stepped off the train. The passengers accept this tentatively, but M. Bouc and Dr. Constantine are unsatisfied, so Poirot offers another one.
He summarizes the inconsistencies of the case and then concludes that each one of the twelve passengers, including the conductor, Pierre Michel, murdered Ratchett, as each one had a personal connection to Daisy Armstrong or the Armstrong household. This explains why the stab wounds appear to have been made by different people: because they were. In addition, the twelve passengers colluded to throw false or irrelevant evidence in Poirot’s path, including the stopped watch, the handkerchief, and the scarlet kimono itself. Poirot thinks that it would take an artist to pull off a scheme like this, and is about to identify Mrs. Hubbard as the actress Linda Arden when she comes forward and admits it. She explains the depths of grief that everyone touched by Ratchett’s crime experienced and relates how the twelve of them planned the murder, bringing on Pierre Michel, the father of Daisy’s French nursemaid, Hardman, the French nursemaid’s lover, and Colonel Arbuthnot, who fought alongside Colonel Armstrong in the war, in addition to those previously identified as connected to the Armstrongs.
Linda Arden asks Poirot what he plans to do, and M. Bouc suggests that Poirot’s first solution to the crime was more credible after all. Poirot agrees, shielding the twelve conspirators from suspicion and arrest.