Hercule Poirot boards the luxurious Orient Express, traveling from Istanbul to London. The train is unusually full, with a diverse group of passengers from different backgrounds. During the journey, a wealthy and unpleasant American named Samuel Ratchett approaches Poirot, asking for protection, as he fears for his life. Poirot refuses.

That night, the train is stopped by a snowdrift, and the next morning, Ratchett is found murdered in his compartment, stabbed twelve times. The door was locked from the inside, making it a locked-room mystery.