Investigation

The investigation into the murder of Samuel Edward Ratchett on the snowbound Orient Express, conducted by Hercule Poirot at the behest of his friend M. Bouc, is the central action of the novel. It transforms a locked-room puzzle into a profound moral and legal dilemma, as Poirot systematically dismantles the passengers' carefully constructed lies to uncover a conspiracy of twelve executioners acting as a self-appointed jury.

Scope and Authority

M. Bouc, a director of the Compagnie Internationale des Wagons Lits, formally places the matter in Poirot's hands, saying, "Take command of this investigation!" Poirot accepts, stating, "C'est entendu. You place the matter in my hands." His first official act is to request a plan of the Istanbul-Calais coach, a note of the occupants of each compartment, and their passports and tickets. He then establishes the restaurant car as his courtroom, where he will interview each passenger. The investigation is confined to the Istanbul-Calais coach, as the train is snowbound and no one can have left it since half an hour after midnight.

Method and Progression

Poirot's method is a blend of meticulous evidence-gathering and psychological deduction. He interviews each passenger, taking their formal evidence, and then searches their luggage. He notes inconsistencies and lies, such as the fact that the Swedish lady, Greta Ohlsson, and the English girl, Mary Debenham, provide alibis for each other, a pattern he finds "extraordinary." He also observes that the English valet and the Italian, and Mr. MacQueen and Colonel Arbuthnot, similarly vouch for each other. This leads him to a crucial realization: "They cannot all be in it!" He then sees the light: "They were all in it." The investigation becomes a process of identifying each passenger's role in the Armstrong household, a task he accomplishes through a series of "guesses" that are in fact deductions from their testimony and behavior.

The Unraveling of the Conspiracy

Poirot's investigation systematically exposes the conspiracy. He reveals that Countess Andrenyi is actually Helena Goldenberg, the younger sister of Mrs. Armstrong, and that her passport has been altered. He forces Mary Debenham to admit she was the governess in the Armstrong household. He extracts confessions from Antonio Foscarelli, the former chauffeur; Greta Ohlsson, the former nurse; and Edward Masterman, the former batman. He deduces that Pierre Michel, the Wagon Lit conductor, is the father of Susanne, the French nursemaid who committed suicide. He identifies Cyrus Hardman as a detective who loved Susanne, and Hildegarde Schmidt as the Armstrongs' cook. Finally, he reveals that Mrs. Hubbard is actually Linda Arden, the famous actress and mother of Mrs. Armstrong. The investigation culminates in Poirot's presentation of two solutions: the first, a convenient fiction of an outside assassin; the second, the truth of a conspiracy of twelve, each of whom stabbed Ratchett once.

Significance and Resolution

The investigation's significance lies not in the discovery of a single culprit but in the revelation of a collective act of vigilante justice. Poirot's role shifts from detective to arbiter of a moral dilemma. He presents both solutions to M. Bouc and Dr. Constantine, who choose the first, more palatable explanation. Poirot then retires from the case, allowing the conspiracy to go unpunished by official law. The investigation thus becomes a vehicle for exploring the conflict between legal justice and moral vengeance, leaving the reader to judge the actions of the twelve executioners.