"The Mysterious Affair at Styles" is the debut detective novel by British author Agatha Christie, introducing Hercule Poirot, Inspector Japp, and Arthur Hastings. Set during World War I, the plot unfolds at Styles Court in Essex, where Hastings is invited to stay by his friend John Cavendish. The household is disrupted when Emily Inglethorp, the wealthy owner of Styles and a patron of Poirot, is found poisoned. Poirot, a meticulous Belgian refugee detective with a knack for understanding human motives, takes on the case at Hastings' request. Through intricate investigation, Poirot discovers that Emily's new husband, Alfred Inglethorp, is central to the case. Despite being the main beneficiary of Emily's will, Alfred's suspicious actions and inability to provide an alibi draw attention. However, the twist unfolds when Poirot reveals that Alfred and his accomplice, Evelyn Howard, conspired to poison Emily by adding a precipitating substance to her regular medicine, leading to an overdose, while also setting up misleading evidence to redirect the investigation. Christie intricately crafts a plot involving family tensions, hidden motives, and multiple suspects, typical of the Golden Age of Detective Fiction, which she helped define. This novel not only launched Christie’s prolific writing career but also introduced Poirot, who would go on to become one of the most famous characters in detective fiction, featured in many of Christie's subsequent works.