Murder on the Orient Express is a detective novel by Agatha Christie. While returning on the Orient Express to London, detective Hercule Poirot meets a misanthropic American named Mr. Ratchett. After Mr. Ratchett is murdered on the train, Poirot is hired to solve the mystery, discovering that Mr. Ratchett is actually the kidnapper and child-killer, Cassetti. He also learns that everyone in coach class had some reason to kill him and, in the end, discovers that everyone was complicit in the murder.
Hercule Poirot, the internationally famous detective, boards the Orient Express (Simplon-Orient-Express) in Istanbul. The train is unusually crowded for the time of year. Poirot secures a berth only with the help of his friend Monsieur Bouc, a director of the Compagnie Internationale des Wagons-Lits . When a Mr. Harris fails to show up, Poirot takes his place. On the second night, Poirot gets a compartment to himself.
During the journey, Poirot is approached by one of the passengers, Mr. Samuel Ratchett, an American businessman, who claims his life is in danger. He produces a small gun that he carries at all times, saying he believes it's necessary. He wants to hire Poirot to discover who is threatening him. Despite offers of increasingly substantial sums of money, Poirot declines Ratchett's offer, saying, "I do not like your face".
That night, in Vinkovci, at about 23 minutes before 1:00 a.m., Poirot wakes to the sound of a scream. It seems to come from the compartment next to his, which is occupied by Mr. Ratchett. When Poirot peeks out his door, he sees the conductor knock on Mr. Ratchett's door and ask if he is all right. A man's voice replies in French, " Ce n'est rien. Je me suis trompé " ("It's nothing. I was mistaken"), and the conductor moves on to answer another bell further down the passage. Poirot decides to go back to bed but is disturbed by the fact that the train is unusually still.
As he lies awake, Poirot hears Mrs. Hubbard ringing the bell urgently. When he rings the conductor for a bottle of mineral water, Poirot learns that Mrs. Hubbard claimed that someone had been in her compartment, and that the train has stopped because a large snowdrift is blocking the track. He dismisses the conductor and tries to go back to sleep, only to be awakened again by a knock on his door. This time, when Poirot gets up and looks out his door, the passage outside his compartment is empty, except for a woman in a scarlet kimono retreating down the passage in the distance. The next day, he awakens to find that Ratchett is dead, having been stabbed 12 times in his sleep. Bouc suggests that Poirot take the case, as he is so experienced with similar mysteries. Nothing more is required than for Poirot to sit, think, and take in the available evidence.
The door to Ratchett's compartment was locked and chained. One of the windows is open. Some of the stab wounds are very deep, at least three are lethal, and some are glancing blows. Furthermore, some of the wounds appear to have been inflicted by a right-handed person and some by a left-handed one. The pistol Ratchett carried is discovered under his pillow, unfired. A glass on the nightstand is examined and revealed to be drugged. A small pocket watch is discovered in Ratchett's pajamas, broken and stopped at 1:15 a.m.
Poirot finds several more clues in the victim's cabin and on board the train, including a woman's linen handkerchief embroidered with the initial "H", a pipe cleaner, and a button from a conductor's uniform. All of these clues suggest that the murderer or murderers were somewhat sloppy. However, each clue seemingly points to different suspects, which suggests that some of the clues were planted.
By reconstructing parts of a burned letter, Poirot discovers that Ratchett was a notorious fugitive from the United States named Lanfranco Cassetti. Five years earlier, Cassetti kidnapped three-year-old American heiress Daisy Armstrong. Although the Armstrong family paid a large ransom, Cassetti murdered the little girl long before the ransom deadline and fled the country with the money. Daisy's mother, Sonia, was pregnant when she heard of Daisy's death. The shock sent her into premature labour, and both she and the baby died. Her husband, Colonel Armstrong, shot himself out of grief. Daisy's nursemaid, Susanne, was suspected of complicity in the crime by the police, despite her protests. She threw herself out of a window and died, only to be found innocent afterwards. Although Cassetti was caught, his resources allowed him to get himself acquitted on an unspecified technicality, although he still fled the country to escape further prosecution for the crime. As the evidence mounts, it continues to point in different directions, giving the appearance that Poirot is being challenged by a mastermind. A critical piece of missing evidence—the scarlet kimono worn the night of the murder by an unknown woman—turns up on top of Poirot's own luggage.
After meditating on the evidence, Poirot assembles Bouc and Dr. Constantine, along with the 13 suspects, in the restaurant car, and lays out two possible explanations of Cassetti's murder. The first explanation is that a stranger— some gangster enemy of Cassetti — boarded the train at Vinkovci, the last stop, murdered Cassetti for unknown reasons, then escaped unnoticed and it's possible that the man has already left Yugoslavia. The crime occurred an hour earlier than everyone thought, because the victim and several others failed to note that the train had just crossed into a different time zone. The other noises heard by Poirot on the coach that evening were unrelated to the murder. However, Dr. Constantine objects, saying that Poirot must surely be aware that this does not explain the circumstances of the case.
Poirot's second explanation is much longer and rather more sensational: all of the suspects are guilty. Poirot's suspicions were first aroused by the fact that all the passengers on the train were of so many different nationalities and social classes, and that only in the "melting pot" of the United States would a group of such different people form some connection with each other.
Poirot reveals that the 13 other passengers on the train, and the train conductor, were all connected to the Armstrong family in some way:
All these friends and relations had been gravely affected by Daisy's murder, and outraged by Cassetti's subsequent escape. They took it into their own hands to serve as Cassetti's executioners, to avenge a crime the law was unable to punish. Each of the suspects stabbed Cassetti once, so that no one could know who delivered the fatal blow. Twelve of the conspirators participated to allow for a "12-person jury", which Arbuthnot approved of. Countess Andrenyi took no part in the crime as she would have been suspected the most, so her husband took her place. One extra berth was booked under a fictitious name– Harris – so that no one but the conspirators and the victim would be on board the coach, and this fictitious person would subsequently disappear and become the primary suspect in Cassetti's murder, a man who was 'dark' and had a 'womanish' voice. Of course there was no such person. (The only people not involved in the plot would be Bouc, for whom the cabin next to Cassetti had already been reserved, and Dr. Constantine). The main inconvenience for the executioners was the snowstorm and the last minute, unwelcome presence of Poirot, which caused complications resulting in several crucialclues being left behind.
Poirot summarises that there was no other way the murder could have taken place, given the evidence. Several of the suspects have broken down in tears as he has revealed their connection to the Armstrong family, and Mrs. Hubbard/Linda Arden confesses that the second theory is correct, but begs Poirot to tell the authorities that she acted alone as Cassetti's murderess. The evidence could be skewed to implicate her and she declares she would gladly go to prison if it meant the other passengers were spared. She points out that everyone present has suffered because of Cassetti's misdeeds, that there would likely have been other victims like Daisy if Cassetti had gone unpunished, and that Colonel Arbuthnot and Mary Debenham are in love. Fully in sympathy with the Armstrong family, and feeling nothing but disgust for the victim, Bouc pronounces the first explanation as correct. Dr. Constantine agrees, saying he will edit his original report of the murder as he now "recognizes" some mistakes he has made, which clearly indicate that Poirot's first explanation was correct, after all. Poirot announces that he has "the honour to retire from the case".