In 1934, renowned Belgian detective Hercule Poirot solves a theft at the Church of the Holy Sepulchre in Jerusalem. Poirot has OCD, seeks balance in life, and considers his case-solving ability to spot lies a curse. His friend Bouc, the nephew of the director of the luxury Orient Express train service, arranges a bunk for him to return to London. Poirot boards the train with Bouc and thirteen other Passengers: American widow Caroline Hubbard, American businessman Edward Ratchett, with his English manservant Edward Masterman 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; Medical Doctor John Arbuthnot; Mary Debenham, a teacher; Pilar Estravados, a Spanish missionary; Cuban-American car salesman Biniamino Marquez; and Gerhard Hardman, an Austrian university professor.
Ratchett offers to hire Poirot to be his bodyguard during the three-day journey, having received anonymous threatening letters. Poirot refuses upon learning that he is a con artist. That night, Poirot hears noises coming from Ratchett's compartment, and sees someone in a red kimono running down the hallway. An avalanche derails the train's engine, stranding the passengers.
The next morning, Poirot discovers Ratchett has been murdered, stabbed a dozen times. Poirot and Bouc investigate the passengers as repairs begin. Evidence indicates that Ratchett was murdered by one person who dealt several blows, and Mrs. Hubbard claims that a man was in her compartment in the night. Poirot discovers a partially burned note connecting Ratchett to the kidnapping of Daisy Armstrong, a child who was abducted from her bedroom and held for ransom. Though the family paid the ransom, Daisy was murdered. Ratchett's true identity is revealed: he was John Cassetti, Daisy's kidnapper and killer. The shock of her death caused her mother Sonia to suffer a fatal miscarriage; her father, Colonel John Armstrong, committed suicide shortly after. The family's nursemaid Susanne was wrongly suspected of complicity, leading to her arrest and subsequent suicide in police custody, only to be found innocent afterward.
More evidence is found, including a bloodstained handkerchief, and, in Mrs. Hubbard's compartment, the button of a conductor's uniform. The red kimono is later found—in Poirot's own suitcase and the conductor's uniform is found in the cabin of one of the testifying passengers. Hubbard is suddenly stabbed in the back; she survives but cannot identify the culprit. Poirot discovers many of the passengers have direct connections to the Armstrong family and uncovers their hidden pasts. While interviewing Debenham, Poirot is shot in the shoulder by Dr Arbuthnot, who claims responsibility for the murder, but Bouc stops him from killing Poirot. Poirot realizes that Arbuthnot—a doctor and former army sniper—never meant to kill him.
Poirot confronts the suspects outside the train, offering 2 theories of how Cassetti died. The first is simple but does not meet all of the facts: An agent of Ratchett's enemies disguised as a conductor boarded the train at a previous stop, murdered Cassetti, and fled at the stop as the train left. The second is more complex: with every single suspect connected to the Armstrongs, Susanne, or her trial in some way, they all had a motive against Cassetti:
MacQueen's father was the district attorney for the kidnapping case, who prosecuted Susanne and sent her to prison, only for his career to be destroyed when the truth was discovered after her suicide
Masterman, was Colonel Armstrong's batman during the war, and later his valet, who also acted as butler to the Armstrong household
Dr. Arbuthnot was Colonel Armstrong's comrade and best friend
Countess Andrenyi (née Goldenberg) was Sonia Armstrong's sister and Daisy's aunt
Count Andrenyi was Sonia's brother-in-law
Princess Dragomiroff was Sonia's godmother, and a friend of her mother
Ms. Debenham was Sonia's secretary and Daisy's governess
Schmidt was the Armstrong family's cook
Marquez was the Armstrong family's chauffeur
Estravado was Daisy's nurse
Hardman, whose real name is Cyrus, was a policeman in love with Susanne
Pierre Michel, the train's conductor, was Susanne's brother
Hubbard is revealed to be Linda Arden, a former stage actress and aspiring director, as well Sonia's mother and Daisy's Grandmother.
Hubbard confirms Poirot's second theory, admitting that she planned the murder and had recruited everyone else to help her. All the other passengers and Pierre Michel took turns stabbing Cassetti. Debenham wore the kimono, and Arbuthnot stabbed Hubbard without endangering her life, to convince Poirot of a lone killer. Poirot challenges the passengers and Michel to shoot him with a confiscated gun since he is the only one who can expose their plot; Bouc can lie, but Poirot, obsessed with truth and balance, cannot. Hubbard grabs the gun and tries to kill herself, but it is not loaded; Poirot wanted to see how the suspects would react and her reaction confirms none of them are truly killers at heart.
With the train back on track, Poirot concludes that justice is impossible in this case, as Cassetti deserved death and none of them deserve to go to jail; for the first time, Poirot will have to live with a lie and imbalance. He presents the lone killer theory to the Yugoslavian police and they accept it, allowing the others to leave on the train. As he disembarks, a British Army messenger asks him to investigate a "murder on the bloody Nile". Poirot accepts the case.
Kenneth Branagh as Hercule Poirot
Tom Bateman as Bouc
Penélope Cruz as Pilar Estravados
William Dafoe as Cyrus Bethman Hardman
Judi Dench as Princess Natalia Dragomiroff
Johnny Depp as Lanfranco Cassetti, alias Edward Ratchett
Josh Gad as Hector MacQueen
Derek Jacobi as Edward Henry Masterman
Leslie Odom Jr. as Dr. Arbuthnot
Michelle Pfeiffer as Linda Arden
Daisy Ridley as Mary Debenham
Marwan Kenzari as Pierre Michel
Olivia Colman as Hildegarde Schmidt
Lucy Boynton as Countess Helena Andrenyi
Manuel Garcia-Rulfo as Biniamino Marquez
Sergei Polunin as Count Rudolph Andrenyi
Phil Dunster as Col. John Armstrong
Miranda Raison as Sonia Armstrong
based on the 1934 novel of the same name by Agatha Christie
the fourth screen adaptation of Christie's novel, following the 1974 film, a 2001 TV film version, and a 2010 episode of the television series Agatha Christie's Poirot