Upon arriving at the Tokatlian Hotel in Istanbul, private detective Hercule Poirot receives a telegram prompting him to cancel his arrangements and return to London. He instructs the concierge to book a first-class compartment on the Orient Express leaving that night. After boarding, Poirot is approached by Mr. Ratchett, a malevolent American he initially saw at the Tokatlian. Ratchett believes his life is being threatened and attempts to hire Poirot but, due to his distaste, Poirot refuses. “I do not like your face, Mr. Ratchett,” he says.
On the second night of the journey, the train is stopped by a snowdrift near Vinkovci. Several events disturb Poirot’s sleep, including a cry emanating from Ratchett’s compartment. The next morning, Mr. Bouc, an acquaintance of Poirot and director of the company operating the Orient Express, informs him that Ratchett has been murdered and asks Poirot to investigate, in order to avoid complications and bureaucracy when the Yugoslav police arrive. Poirot accepts.
After examining the body and Ratchett’s compartment, Poirot ascertains Ratchett’s real identity and possible motives for his murder. A few years before, in the United States, three-year-old heiress Daisy Armstrong was kidnapped by a man named Cassetti. Cassetti eventually killed the child, despite collecting the ransom from the wealthy Armstrong family. The shock devastated the family, leading to a number of deaths and suicides. Cassetti was caught, but fled the country after he was acquitted. It is suspected that Cassetti used his considerable resources to rig the trial. Poirot concludes that Ratchett was, in fact, Cassetti.
As Poirot pursues his investigation, he discovers that everyone in the coach had a connection to the Armstrong family and, therefore, had a motive to kill Ratchett. Poirot proposes two possible solutions, leaving it to Bouc to decide which solution to put forward to the authorities. The first solution is that a stranger boarded the train and murdered Ratchett. The second solution is that all 13 people in the coach were complicit in the murder, seeking the justice that Ratchett had averted in the United States. He concedes Countess Andrenyi did not take part, so the murderers numbered 12, resembling a self-appointed jury. Mrs. Hubbard, revealed to be Linda Arden, Daisy Armstrong’s grandmother, confesses that the second solution is correct.