Famous detective Hercule Poirot is called back from Istanbul to London on urgent business. He intends to book a first-class compartment on the Orient Express, run by his former friend and colleague, Monsieur Bouc. The train is surprisingly full, but Bouc manages to secure Poirot a spot in the first-class cabin.

While aboard the Orient Express, Poirot meets a host of peculiar characters: an aging Russian princess, her Swedish companion, a Hungarian countess, a Minnesota housewife, a Scottish colonel, an English governess, a French conductor, a disagreeable American businessman, and his anxious secretary. While aboard the train, the angry businessman, Samuel Ratchett, corners Poirot and demands Poirot to investigate a series of ominous letters that have been sent to Ratchett, threatening his life.