The Mysterious Affair at Styles is a detective novel by Agatha Christie. It was written in 1916 and was first published by John Lane in the US in October 1920 and in the UK by The Bodley Head (John Lane's UK company) on January 21 1921. The US edition retailed at $2.00 and the UK edition at seven shillings and sixpence .

It is Christie's first published novel, and introduces Hercule Poirot, Inspector (later, Chief Inspector) Japp and Lieutenant Hastings (later, Captain). The story is told in first person by Hastings, and features many of the elements that, thanks to Christie, have become icons of the Golden Age of Detective Fiction. It is set in a large, isolated country manor. There are a half-dozen suspects, most of whom are hiding facts about themselves. The book includes maps of the house, the murder scene, and a drawing of a fragment of a will. Also, there are a number of red herrings and surprise plot twists.