After successfully concluding a case in the Middle East, detective Hercule Poirot (Albert Finney) sets off for England aboard the famed Orient Express. During this journey, he reunites with his friend Bianchi (Martin Balsam), who is the director of the Compagnie Internationale des Wagons-Lits, and notices that the train is unusually full for the season, with all first-class cabins booked.
Just as the train departs Istanbul, a wealthy American businessman named Ratchett ([Richard Widmark]) approaches Poirot, offering a hefty sum of $15,000 for his services, as he has been receiving death threats. However, Poirot dismisses the case as uninteresting and declines the offer. As night falls, the train becomes ensnared in a fierce snowstorm in the Balkans. The following morning, the shocking news emerges: Ratchett has been found brutally killed in his cabin.
Determined to uncover the truth, Poirot, alongside Bianchi, enlists the help of Dr. Constantine ([George Coulouris]), a Greek doctor traveling unnoticed in another coach, along with Pierre Michel ([Jean-Pierre Cassel]), the middle-aged French conductor who is also among the suspects but aids in the investigation. As they delve deeper, Poirot uncovers a shocking revelation: Ratchett was not who he seemed; in reality, he was a notorious gangster named Cassetti who had a dark past involving the kidnapping and murder of a young girl named Daisy Armstrong.
An examination by Dr. Constantine indicates that Ratchett had been stabbed twelve times, with evidence suggesting that the murder occurred around 1:15 a.m. while the train was trapped in snow. With the other train compartments locked and no footprints in the snow, it quickly becomes apparent that the murderer is likely still present among the passengers.
As Poirot investigates further, he discovers that all the passengers have ties to the Armstrong family, each harboring motives for revenge due to the tragic aftermath of Daisy's murder. The suspects include:
- Hector McQueen ([Anthony Perkins]), Ratchett's secretary;
- Edward Beddoes ([Sir John Gielgud]), his British valet;
- Mary Debenham ([Vanessa Redgrave]), a British teacher returning from Baghdad;
- Colonel Arbuthnott ([Sean Connery]), an officer in the British Indian Army;
- Princess Natalia Dragomiroff ([Wendy Hiller]), an elderly Russian royal;
- Hildegarde Schmidt ([Rachel Roberts]), the Princess’s maid;
- Count Rudolf Andrenyi ([Michael York]), a Hungarian diplomat;
- Countess Elena Andrenyi ([Jacqueline Bisset]), his wife;
- Greta Ohlsson ([Ingrid Bergman]), a Swedish missionary;
- Mrs. Harriet Belinda Hubbard ([Lauren Bacall]), a talkative American socialite;
- Antonio Gino Foscarelli ([Denis Quilley]), an Italian car salesman;
- Cyrus B. “Dick” Hardman ([Colin Blakely]), a detective masquerading as a talent agent;
- Pierre-Paul Michel ([Jean-Pierre Cassel]), the conductor of the sleeping car.
As Poirot interrogates each suspect, he uncovers painful connections to the Armstrong tragedy. It becomes clear that several of them had personal stakes in the revenge against Cassetti for the havoc he caused in their lives. Eventually, Poirot reveals that Cassetti had been sedated by Beddoes and McQueen, allowing all present to take turns delivering the fatal wounds.
However, Poirot presents two theories to the remaining passengers: the complex murder in which each suspect played a role, and a simpler explanation involving a potential intruder. He suggests that Cassetti may have been embroiled in a Mafia feud, leading to his demise. Bianchi chooses the less complex explanation for the local police, deeming that Cassetti ultimately got what he deserved. Poirot grapples with his conscience but finds solace in the belief that justice has been served.
