S02E10 - The Mysterious Affair at Styles

The novel is set in England during World War I at Styles, an Essex country manor (also the setting of Curtain, Poirot's last case). The manor is owned by the wealthy widow Emily Inglethorp, who has recently remarried to a much younger man, Alfred Inglethorp. 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 of what proves to be strychnine poisoning. Captain Hastings, a house guest, enlists the help of his friend Hercule Poirot, who is staying in the nearby village, Styles St. Mary. Poirot pieces together events surrounding the murder. 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. She ate little at dinner and retired early to her room with her document case. The case was later forced open by someone and a document removed. Alfred Inglethorp left Styles earlier in the evening and stayed overnight in the nearby village, so was not present when the poisoning occurred. Nobody can explain how or when the strychnine was administered to Mrs. Inglethorp.