Famous crime novelist Ariadne Oliver is invited to a Halloween party hosted by Rowena Drake for local teenagers in the small town of Woodleigh Common. At the party, one teenager, Joyce Reynolds, claims she witnessed a murder in the past. Later, Joyce is found drowned, and Ariadne asks her friend Hercule Poirot to investigate. Poirot assumes Joyce was killed for what she said and must determine if her claim was true, meaning he would have two deaths to solve instead of just one.