The Halloween Party (Haunted Venice) by Agatha Christie

"I saw a murder!" - declared thirteen-year-old Joyce during a Halloween party. Of course, none of the guests believed her, not even Ariadne Oliver, a famous detective author. Oh, these teenagers, who are always trying to attract attention in any way!

However, when the guests leave, the girl is found murdered. A shocked Mrs. Oliver turns to her friend Hercule Poirot for help.

They face a double mystery: what crime did Joyce witness and who silenced her forever? And here Poirot finds the first clue.