In the novel Hallowe'en Party, Joyce Reynolds was a 13 year-old girl, known for not being entirely truthful. She was living in Woodleigh Common and attended a halloween party at Apple Trees organised by Rowena Drake.
During the party, she claimed that she once had witnessed a murder some time ago. Asked why she did not go to the police, she said she was "quite young at the time". She didn't know it was a murder at the time she saw it. Then something somebody said about a month or two before the party made her think that what she saw must have been a murder. The other people at the party, such as her elder sister Ann all thought she was just boasting, especially to impress the crime writer Ariadne Oliver who was at the party.
Subsequently, at the end of the party, Joyce was found drowned in a bucket of water which had been set up as part of a "bobbing for apples" game. This event launched Poirot's investigation which would form the main plot of the book.