The story is about the Ashbys, an English country-squire family. Their centuries-old family estate is Latchetts, in the fictional village of Clare, near the south coast of England. It takes place in the late 1940s, after World War II.
The Ashby family consists of Beatrice Ashby ("Aunt Bee"), a spinster of about 50, and the four children of her late brother Bill: Simon, 20; Eleanor, 18–19 and twins Jane and Ruth, 9. Bill and his wife Nora died eight years before. Since then, the Ashbys have been short of money. Bee has kept the estate going by turning the family stable into a profitable business, combining breeding, selling and training horses with riding lessons. When Simon turns 21, he will inherit Latchetts and a large trust fund left by his mother. Simon had a twin brother, Patrick, older than him by a few minutes but soon after Bill and Nora died, Patrick disappeared, leaving what was taken as a suicide note.
The title character, Brat Farrar, is a young man recently returned to England from America. He was a foundling. At the age of 13, the orphanage placed him in an office job but he ran away instead. He ended up in the western US, where he worked at ranches and stables for several years and became an expert horseman until a fall injured his leg and he was left with a limp.
On a street in London, someone completely unknown to Brat greets Brat as "Simon". This stranger is Alec Loding, a second-rate actor. He knows the Ashby family intimately and sees a way to help his own fortunes. Alec's idea is that Brat should impersonate the missing twin, Patrick and as the elder brother, claim the trust and the estate. Alec remembers a great deal about the Ashbys, Latchetts, and the village, which will allow him to coach Brat on all the background details. In return Brat will give him a share of the money. Brat is reluctant but eventually agrees, especially when he hears about the horses.
After two weeks of tutoring, Brat appears at the office of the Ashby family solicitor, saying he adopted the name "Brat Farrar" after running away. He gives his own story as the account of Patrick's missing years. Mr. Sandal informs Bee, who meets Brat and is also convinced. Over the next two weeks, Sandal verifies Brat's story. The family receives "Patrick" at Latchetts. His presence leads to the discovery of Patrick's actual fate, murdered by Simon. The final confrontation leaves Simon dead and Brat in hospital. There Bee's Uncle Charles identifies Brat as an illegitimate son of Bee's wastrel cousin Walter.
A version was produced in 1950 and shown on television, as of a series called "The Philco-Goodyear Television Playhouse".
The novel was loosely adapted in 1963 by Hammer Films as Paranoiac, with some of the details changed: the Ashbys are wealthy by other means with no money problems, and no need to raise horses; the impostor who plays Tony is not a long-lost cousin; nor does the character of Uncle Charles appear in the film.