A murder investigation aboard the Orient Express, led by the renowned detective Hercule Poirot. After a wealthy American, Mr. Ratchett (later identified as Cassetti, a notorious kidnapper and murderer), is found stabbed to death, the train is stranded in a snowdrift, making an external culprit impossible. Poirot, with the assistance of M. Bouc and Dr. Constantine, interviews various passengers and uncovers a complex web of lies and concealed identities, many of whom are connected to Cassetti's past crimes. The investigation reveals that the murder was a meticulously planned act of collective revenge, with multiple individuals participating to ensure Cassetti's death, culminating in the revelation that the "innocent" conductor was also involved, making it a "perfect" crime with no single guilty party.