The narrative documents Hercule Poirot and his associate Captain Hastings intervening in a seemingly accidental series of events surrounding the young, modern owner of End House, Nick Buckley. After Poirot witnesses a bullet narrowly miss Nick, the detectives learn she is the potential target of a murderer, prompting her to invite her cousin Maggie Buckley for protection. This precaution tragically fails when Maggie is killed during a local fireworks display, having been mistaken for Nick while wearing her distinctive shawl. The subsequent investigation uncovers the critical fact that Nick's recently deceased fiancé, Michael Seton, was a millionaire's heir, establishing an enormous financial motive for the killer. Employing subterfuge, Poirot eventually exposes the astonishing truth: Nick Buckley was the criminal, having orchestrated the "accidents" and murdered her own cousin, who was the rightful secret recipient of Seton's vast fortune.