When Lucy Angkatell invites Hercule Poirot to a summer gathering at her home, her guests decide to stage a mock murder by the swimming pool as a prank. The game turns deadly when victim John Christow collapses with genuine wounds, his final whispered word—"Henrietta"—echoing across the poolside before a gun vanishes beneath the water. Poirot must unravel a web of secrets and motives among the assembled guests to identify which among them turned jest into tragedy.
