The novel features Belgian detective Hercule Poirot and the mystery novelist Ariadne Oliver. A girl at a Hallowe'en party with Ariadne Oliver in attendance, claims she witnessed a murder, which at the time as she was so young, she had not realized was a murder. Soon, the girl is found murdered. Oliver calls in Poirot to find the cause of this and the next murder. This book was dedicated to P.G. Wodehouse.
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.