Hill estate. Richard Devonport has summoned him to prove that his fiancee, Helen, is innocent of the murder of his brother, Frank. There is one strange condition attached to this request: Poirot must conceal his true reason for being there from the rest of the Devonport family. On the coach, a distressed woman leaps up, demanding to disembark. She insists that if she stays in her seat, she will be murdered. A seat-swap is arranged, and the rest of the journey passes without incident. But Poirot has a bad feeling about it, and his fears are later confirmed when a body is discovered in the Devonports' home with a note that refers to 'the seat that you shouldn't have sat in'. Could this new murder and the peculiar incident on the coach be clues to solving the mystery of who killed Frank Devonport? And can Poirot find the real murderer in time to save an innocent woman from the gallows?