It is wartime England, and Captain Arthur Hastings, recovering on sick leave, accepts an invitation to Styles Court, the Essex country house of his friend John Cavendish. The estate belongs to John's wealthy stepmother, Emily Inglethorp, whose recent marriage to a younger man, Alfred, has unsettled the household. When Emily dies in agony one night, locked in her room, the cause proves to be poison, and almost everyone under the roof has a motive. Hastings learns that a Belgian refugee staying nearby is none other than Hercule Poirot, a retired police detective whose fastidious habits and faith in order and method conceal a formidable mind. Poirot agrees to investigate. Suspicion shifts from the devoted companion to the resentful stepsons, the secretive daughter-in-law, the young dispensary worker, and a visiting toxicologist, while inquests, withheld letters, and a charred fragment of paper complicate every theory. As Poirot sifts evidence that the local police misread, Hastings narrates a case built on small inconsistencies and precise timing. The first outing for one of detective fiction's most enduring partnerships, the story unfolds as a closed-circle puzzle in which the smallest domestic detail may decide who hangs and who walks free.