Murder on the Orient Express begins with Hercule Poirot boarding the Orient Express in Constantinople after solving a significant case in Palestine. On the second night of his journey, Poirot is awakened by loud noises and later finds out that the train has been stopped due to a snowstorm.
Subsequently, the next morning Poirot finds out that Mr. Ratchett, who had occupied the compartment next to his, had been murdered by being stabbed twelve times in his sleep. Poirot is then called on to solve the murder and he begins his work at once, gathering evidence and listing down suspects.
As he probes into the murder, Poirot finds out that the victim, Samuel Edward Ratchett, is a man with a shady past. He learns that Ratchett, known as Cassetti in the USA, had fled from the country after having being involved in the kidnapping and murder of Daisy Armstrong, a three-year-old American heiress. Poirot then discovers how Daisys death triggered the subsequent death of almost her entire household and soon, using his detective skills, he catches the culprit behind Ratchetts murder.