In 1934, the renowned detective Hercule Poirot (Kenneth Branagh) finds himself entangled in a perplexing theft case at the Church of the Holy Sepulchre in Jerusalem. A man marked by his obsessive-compulsive tendencies, Poirot strives for balance in his life and views his extraordinary skill to uncover lies amid truths as somewhat of a curse. While yearning for some peace in Istanbul, he is compelled to return to London to tackle yet another mystery. His close friend, Bouc (Tom Bateman), who manages the Orient Express, extends an invitation for him to stay aboard the illustrious train.
Amidst this setting, businessman Samuel Ratchett (Johnny Depp) solicits Poirot's services as a bodyguard during their three-day voyage, fearing threats he has received from an anonymous source. Poirot, however, declines this job. That very night, he becomes alert to peculiar sounds emanating from Ratchett's cabin and witnesses a mysterious figure clad in a red kimono dashing down the corridor. Suddenly, tragedy strikes when an avalanche derails the train, trapping the passengers in its grip.
The following morning unveils the shocking news that Ratchett has been brutally murdered, stabbed multiple times during the night. Teaming up with Bouc, Poirot begins to unearth the identities and alibis of the passengers as repairs to the train get underway. Evidence points towards a single perpetrator responsible for Ratchett's demise, with Caroline Hubbard (Michelle Pfeiffer) asserting that someone had entered her compartment, providing a potential route for the murderer into Ratchett's space.
Poirot's investigation reveals a partially ruined note linking Ratchett to the infamous kidnapping of Daisy Armstrong, a little girl who was taken from her home for ransom and later discovered dead despite the payment being made. Ratchett is exposed as John Cassetti, the very man responsible for Daisy's abduction and murder. The heartbreaking repercussions of Daisy's death led to her mother's tragic demise during childbirth and her father's subsequent suicide. The case deepens further with the shocking revelation of the nursemaid Susanne, who was wrongfully accused and later took her own life after being unjustly incarcerated.
The train compartments were securely locked that night, leaving only the individuals within that compartment as potential suspects. Even a guard confirmed he saw no one enter Ratchett’s cabin during the crucial time. An analysis of Ratchett’s coffee indicates it was drugged, yet his valet Henry remains in the dark about how it occurred before or after it was served to Ratchett. As new evidence surfaces, including a bloodied handkerchief and a button from a conductor's uniform found in Hubbard's room, the mystery intensifies further. Hubbard gets attacked but survives, leaving Poirot grasping at the threads of a complex narrative.
Not long after, Poirot engages in interviews with various suspects, including Mary Debenham (Daisy Ridley), where tension escalates when John Arbuthnot nearly shoots Poirot, only to clarify his intent was not to kill but to assert a point. With each interaction, hidden connections to the Armstrong family emerge, weaving a dense tapestry that Poirot meticulously unravels.
Eventually, he confronts the passengers with two potential theories regarding Ratchett’s death. The simpler one suggests that an outsider masquerading as the conductor carried out the murder, while the more intricate theory indicates that all present had a personal connection to the Armstrong tragedy, yielding a collective motive against Cassetti. In a stunning turn, it becomes evident that Hubbard is actually Linda Arden, a former celebrated actress and mother to Sonia Armstrong, who orchestrated the murder plot alongside the other suspects.
Poirot further uncovers deep links among the passengers to the Armstrong family. Hector Willard MacQueen (Josh Gad), Ratchett’s loyal secretary, bears connections through his father to the kidnapping case, fueling his rage against Cassetti. The plot thickens with revelations about Edward Henry Masterman, Colonel Armstrong’s batman and later servant, and Colonel Arbuthnot, a trusted friend of Armstrong. They all share intertwined histories of grief and vengeance stemming from the past.
Ultimately, as Poirot pieces together this complex puzzle, he challenges the suspects, proposing that all of them took part in Ratchett’s death, each delivering a blow to reserve their collective secret. Tragically, at the conclusion of this moral dilemma, Poirot realizes that true justice will elude him. Cassetti's crimes justified his death, leaving Poirot to bear the weight of a lie on his mind. He presents a solitary killer theory to the authorities, facilitating the passengers’ escape. As the train gets back on its course, a messenger approaches Poirot, requesting his expertise on a death that has occurred on the Nile, prompting Poirot to take on yet another complex investigation.
