The story begins during World War I, when Captain Arthur Hastings returns to England from the front and is invited to stay at the home of his friend, John Cavendish, and his wife, Emily, at Styles Court, a grand country house in Essex. Emily Cavendish is a wealthy woman who is known for her large fortune and her complex family relationships. She has remarried, and her second husband, Alistair Ravenscroft, is younger and seemingly more interested in her wealth than in her well-being. Emily's two stepsons, John and Lawrence Cavendish, also live at the estate, along with the family’s staff.

One night, Emily is poisoned and dies suddenly after drinking from a glass of milk. The circumstances surrounding her death are suspicious, and Captain Hastings, who is staying at the estate, seeks the help of his old friend Hercule Poirot, whom he had met during the war while Poirot was working as a Belgian refugee in England.