During the First World War, Arthur Hastings is on sick leave from the Western Front and is invited to stay at Styles Court in Essex by his old friend John Cavendish. On the morning of 18 July, the Styles household wakes to the discovery that Emily Inglethorp, the elderly and wealthy owner, has died. She had been poisoned with strychnine. Hastings ventures out to the nearby village of Styles St Mary to enlist help from his friend Hercule Poirot. Emily's household includes her husband, Alfred Inglethorp, a younger man she recently married; her stepsons (from her first husband's previous marriage) John and Lawrence Cavendish; John's wife Mary Cavendish; Cynthia Murdoch, the daughter of a deceased friend of the family; and Evelyn Howard, Emily's companion.
Poirot learns that, on Emily's death, John is to inherit the manor property, in accordance with his father's will. However, her money will be distributed according to her own will, which she changes at least once a year; her most recent will favours Alfred, who will now inherit her fortune.[15] On the day of the murder, Emily had been arguing with someone, suspected to be either Alfred or John. She had been distressed after this and apparently made a new will, but no one can find any evidence of the new will. Alfred left the manor early that evening and stayed overnight in the village. Meanwhile, Emily ate little at dinner and retired early to her room, taking her document case with her; when her body was found, the case had been forced open. Nobody can explain how or when the poison was administered to her.
Inspector Japp, the investigating officer, considers Alfred to be the prime suspect, as he gains the most from his wife's death. Poirot notes that Alfred's behaviour is suspicious during the investigation. He refuses to provide an alibi and denies purchasing the strychnine in the village, despite evidence to the contrary. Although Japp is keen to arrest him, Poirot intervenes by proving he could not have purchased the poison; the signature for the purchase is not in his handwriting. Suspicion now falls on John, next to gain from Emily's will and without an alibi for the murder. Japp soon arrests him: the signature for the poison is in his handwriting; a phial that contained the poison is found in his room; a false beard and a pair of pince-nez identical to Alfred's are found within the manor.
Poirot's investigations exonerate John of the crime. He establishes that the murder was committed by Alfred Inglethorp, with aid from his cousin Evelyn Howard. The pair pretended to be enemies but were romantically involved. They added bromide, obtained from her sleeping powder, to Emily's regular evening medicine. This caused the low level of strychnine in the medicine to precipitate to the bottom of the bottle, making the final dose lethal. The pair then left false evidence that would incriminate Alfred, which they knew would be refuted at his trial; once acquitted, he could not be tried for the crime again if genuine evidence against him was found, under the law of double jeopardy. The pair framed John as part of their plan; Evelyn forged his handwriting, and the evidence against him was fabricated.
Poirot explains that he prevented Japp from arresting Alfred because Poirot could see that Alfred wanted to be arrested. Thanks to a chance remark by Hastings, Poirot finds a letter in Emily's room that detailed Alfred's intentions for Evelyn. Emily's distress on the afternoon of the murder was because she had found this letter in Alfred's desk while searching for stamps. Emily's document case was forced open by Alfred when he realised she had the letter. He then hid the letter elsewhere in the room to avoid being found with it.
