Hercule Poirot is visiting Istanbul, but is called back to London to work on a case. Poirot's old acquaintance Xavier Bouc, who is director of the Orient Express, secures him a last-minute ticket on the impressive train. The train is completely booked, carrying an eclectic group of passengers such as Princess Dragomiroff and her nervous maid, Hildegarde Schmidt, English governess Mary Debenham and Swedish missionary Greta Ohlsson.
During the journey, ruthless American businessman Samuel Ratchett approaches Poirot. Ratchett cryptically explains to Poirot that he needs to give something back before he can be forgiven, but fears he'll be murdered in the meantime. He asks Poirot for protection, offering a large sum of money, but Poirot refuses. The next morning, Poirot awakens to find the train stuck in a snowdrift and Ratchett dead.
Aided by Dr. Constantine, Poirot examines the murder scene and finds a variety of confusing clues. He interviews those who last saw Ratchett alive: his manservant Edward Masterman, personal assistant Hector MacQueen, and conductor Pierre Michel before making a striking realization about Ratchett. The evidence points towards a particular explanation for the murder, but Poirot also considers a darker, alternative theory. Isolated by the snow and potentially on the train with a killer, Poirot is embroiled in one of the most vexing cases of career, and will soon be forced to make one of the hardest decisions of his life.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
While investigating a case in Palestine for the British army, Hercule Poirot inadvertently incites a young officer to commit suicide when the man is caught in a lie. This pointless tragedy causes Poirot to begin reflecting on the nature of justice — a concern that deepens when he witnesses the stoning of a woman accused of adultery in Istanbul.
It is in this state of mind that he boards the Calais coach of the Orient Express upon getting an urgent request to return to London. Unusually for the time of year, the sleeping car is fully booked, and Poirot spends his first night sharing a cabin with Hector McQueen, the personal assistant to a disagreeable businessman named Samuel Ratchett. The next evening Poirot gets his own cabin, which happens to adjoin Ratchett's. Late that night, Poirot hears a cry of anguish from next door. Out in the corridor, Michel, the conductor, ask if anything is wrong. A voice from Ratchett's cabin replies that all is well.
A little over an hour later the train comes to a screeching halt when it hits a deep snow drift. The next morning, with the vehicle marooned in a winter landscape, the Calais coach is in an uproar: Ratchett has been found murdered!
The director of the train line, who is on board, asks Poirot to wrap up the investigation before the Yugoslav police arrive and take a brutal approach to the case. With assistance from Dr. Constantine, a passenger, Poirot begins.
First of all, there is the nature of Ratchett's wounds: 12 knife punctures made from all directions and to different depths, as if the murderer was switching the knife from hand to hand, while stabbing with varying force.
Then there are these clues: a burned fragment of paper reading "AISY ARMS," a women's handkerchief with the monogram "H," and a button from a conductor's uniform. There are also anonymous threatening notes sent to Ratchett, and the fact that most of the $200,000 in cash that he was carrying is gone.
McQueen's description of his employer's shady background, combined with the "AISY ARMS" clue, leads Poirot to the sudden insight that Ratchett was none other than Lanfranco Cassetti, the kidnapper-murderer of an American girl named Daisy Armstrong in a notorious crime five years earlier. Daisy's parents paid $200,000 in ransom for her release before her body was discovered. The tragedy led to the death of her parents — to shock and suicide — and to the suicide of a housemaid who was wrongly accused by police.
From here, the evidence slowly builds that different passengers on the Calais coach have connections to the Daisy Armstrong case. McQueen admits that his father was the disgraced prosecutor who allowed Cassetti to go free. A lady who calls herself Mrs. Hubbard turns out to be the celebrated actress Linda Arden, Daisy's grandmother. The imperious Princess Dragomiroff volunteers that she was the godmother of Daisy's mother. The princess's maid, Hildegard Schmidt, was the Armstrong's family cook. Unbeknownst to Ratchett/Cassetti, his butler, Edward Masterman, was Daisy's father's orderly in the army, and the passenger Colonel Arbuthnot was his wartime best friend. Furthermore, the colonel's companion, Mary Debenham, was Daisy's governess.
And that's not all. Daisy's aunt (Countess Andrenyi) is aboard. So is her nurse (Greta Ohlsson), her family's chauffeur (Antonio Foscarelli), and her mother's obstetrician (Dr. Constantine). Indeed, with the exception of Poirot and the director of the train line, every single passenger on the Calais coach, plus the conductor Michel (who is the father of the wrongly accused housemaid), has an Armstrong family link — and a motive to want Ratchett/Cassetti dead.
Poirot is therefore left with two theories of the case: either it was an underworld hit for the $200,000; or 12 vengeful Armstrong partisans conspired to take the Orient Express on a night when they knew their victim would be aboard and, acting as jury and executioner, lined up to take turns with the fatal knife. The latter theory has the advantage that all 12 suspects have confessed!
Nonetheless, in meeting with the Yugoslav police, Poirot ponders the paradoxes of justice and informs them that, unfortunately, the hit man was able to escape in the snow.