When Ariadne Oliver attends a children's Hallowe'en party in Woodleigh Common with her friend Judith, a young girl named Joyce Reynolds boasts of having witnessed a murder years before. Joyce's story is heard by all the party, including her strange brother Leopold, her bookworm son Edmund, her smart hostess Rowena Drake, Cottrell, the local clergyman, Mrs Whittaker, the organist of the parish church, and Frances Drake, Rowena's daughter. Some of them dismiss her story, but later that evening Joyce is found dead, drowned in an apple-bobbing bucket in the library. It seems any one of the guests could have slipped out in the dark during a game of Snapdragon and drowned her.
At the request of Ariadne Oliver, Poirot travels down to Woodleigh Common to investigate the murder. Alhough Inspector Raglan of the local police dismisses Joyce's claim, and even her step-mother, Mrs Reynolds, says her daughter was a liar, Poirot takes Joyce's story seriously. Mrs Goodbody, a gossiping charwoman, tells Poirot there have been a number of suspicious deaths in the village in recent years which Joyce could indeed have witnessed: a drowned school teacher, a stabbed solicitor's clerk, and the death of a rich old lady. Mrs Goodbody and the affable gardener, Garfield, are convinced that old curses still haunt the village.
While Poirot is busy piecing together the facts, another child is found drowned in a river. How do a forged codicil to a Will, a missing au pair girl and a secret love affair relate to the death of Joyce? Poirot hopes that Fullerton, Mrs Llewelyn-Smythe's old solicitor, holds the key to the mystery. He realizes he needs to act quickly to save Judith's daughter Miranda, who is also in danger.