During the preparation of a Hallowe'en party in Woodleigh Common, thirteen-year old Joyce Reynolds tells everyone, including Ariadne Oliver, that she saw a murder once, but only recently realised that it was a murder that she had seen. At the end of the party, Joyce is found drowned in an apple-bobbing tub. Mrs Oliver is visiting her friend Judith Butler, whose daughter Miranda (age 12) is too ill to attend the party. With Mrs Oliver's help, Poirot must unmask the real evil of the night.