Set in the summer of 1917 in an Essex country estate, the story follows the war-wounded Captain Arthur Hastings to the Styles St. Mary manor of his friend John Cavendish. The Cavendish household is wrought with tension due to the marriage of John's widowed old aunt Emily, she of a sizeable fortune, to a suspicious younger man, Alfred Inglethorp, twenty years her junior. Emily's two stepsons, John and Lawrence Cavendish, as well as John's wife Mary and several other people, also live at Styles. Late one night, the residents of Styles wake to find Emily Inglethorp dying. When Emily's sudden heart attack is found to be attributable to strychnine, Hastings, who had runs into his old friend, the Belgian Hercule Poirot, he recruits him to aid in the local investigation. With impeccable timing, Hercule Poirot, the insightful retired detective, makes his dramatic entrance to solve a most baffling case.

Who poisoned the wealthy Emily Inglethorpe, and how did the murderer penetrate and escape from her locked bedroom? Suspects abound in the quaint village of Styles St. Mary--from the heiress's fawning new husband to her two stepsons, her volatile housekeeper, and a pretty nurse who works in a hospital dispensary. On the day she was killed, Emily Inglethorp was overheard arguing with someone, most likely her husband, Alfred, or her stepson, John. Afterwards, she seemed quite distressed and, apparently, made a new will--which no one can find. Nobody can explain how or when the strychnine was administered to Mrs. Inglethorp. High on Poirot's list of suspects are: John Cavendish, the elder stepson; Mary Cavendish, his wife; Lawrence Cavendish, the younger stepson; Evelyn Howard, Mrs. Inglethorpe's companion; Cynthia Murdoch, her protegee; and Dr. Bauerstein, a mysterious stranger who lives in Essex. All have motive and opportunity but only Poirot can discover the truth.