A dream honeymoon cruise for Simon and Linnet Doyle turns into a nightmare when Linnet is found shot dead. One of the ship's passengers is Jacqueline de Bellefort, Simon's jealous ex-fiance, and she has already confessed to Hercule Poirot that she wants revenge for Linnet stealing Simon from her.

However, when two more murders are committed, Hercule Poirot, with the help of his friend Colonel Race, begins to see that no one on the ship is as straightforward as they first seemed.