In the 1930s, famed Belgian detective Hercule Poirot boards the luxurious train traveling from Istanbul to Calais. The train is filled with a motley group of passengers, each seemingly with their distinct, unrelated backgrounds. During the journey, Poirot is approached by a wealthy American named Ratchett, who claims to be receiving threatening letters and seeks Poirot's protection, offering a substantial payment for his services. Poirot, sensing Ratchett's unsavory demeanor, declines the offer. Later that night, the train is halted due to a snowdrift, and Ratchett is found murdered in his compartment, having been stabbed multiple times.

With the train stuck and a murderer among the passengers, Poirot is compelled to investigate. He meticulously interviews each passenger and begins uncovering hidden connections and secrets that tie them to the crime. As Poirot delves deeper, he realizes that nearly everyone onboard had a motive for wanting Ratchett dead. Through his keen observational skills and methodical approach, Poirot starts piecing together the puzzle, leading him to a complex and morally challenging conclusion.