Warning: Wikipedia contains spoilers.
The story opens with the novelist Ariadne Oliver attending a Halloween party for teenagers given by a widow, Rowena Drake. During this party Joyce Reynolds, a rather unpleasant girl, boasts that "I saw a murder once [but that] I didn't know it was a murder when I saw it." Later that evening Joyce is found drowned in the bucket used for bobbing for apples. Mrs Oliver summons Hercule Poirot to solve the case. Poirot eventually discovers that Joyce had not seen a murder, but that she was just repeating the words of another teenage girl, Miranda Butler. Miranda had seen Rowena and Michael Garfield murder an au pair girl, Olga Seminoff, as part of a scheme to inherit a large fortune.