The Mysterious Affair at Styles is a detective novel by Agatha Christie. It was written in the middle of World War I, in 1916, and first published by John Lane in the United States in October 1920 and in the United Kingdom by The Bodley Head (John Lane's UK company) on 21 January 1921. The US edition retailed at US$2.00 and the UK edition at seven shillings and sixpence (7/6).

Styles was Christie's first published novel, introducing Hercule Poirot, Inspector (later, Chief Inspector) Japp, and Arthur Hastings. Poirot, a Belgian refugee of the Great War, is settling in England near the home of Emily Cavendish, who helped him to his new life. His friend Hastings arrives as a guest at her home. When the woman is killed, Poirot uses his detective skills to solve the mystery. This is also the setting of Curtain, Poirot's last case.

The book includes maps of the house, the murder scene, and a drawing of a fragment of a will.

Golden Age of Detective Fiction

The story is told in the first person by Hastings, and features many of the elements that have become icons of the Golden Age of Detective Fiction, largely due to Christie's influence. It is set in a large, isolated country manor. There are a half-dozen suspects, most of whom are hiding facts about themselves. The plot includes a number of red herrings and surprise twists.