In the English village of Clare, the Ashby family lives at Latchetts, a historic yeoman estate. Beatrice ("Bee") Ashby has served as guardian to her late brother Bill's four children since Bill and his wife Nora were killed in a plane crash eight years ago. The eldest, Simon, is approaching twenty-one, when he will inherit his mother's fortune. His twin brother, Patrick, apparently committed suicide at thirteen, shortly after their parents' deaths, leaving behind a note that read, "I'm sorry, but I can't bear it any longer. Don't be angry with me." His coat and the note were found on a cliff-top, but his body was never recovered. The younger children are Eleanor, who helps run the stables, and the nine-year-old twins, Ruth and Jane.
Bee has kept Latchetts solvent through its horse-breeding operation. She confides to her friend Nancy Peck, the Rector's wife, that she has always feared Patrick regretted his decision too late, believing he swam out to sea rather than jumping from the cliff. Nancy mentions her brother Alec Loding, a minor London stage actor, who should be invited to Simon's coming-of-age celebration.
Loding, however, has his own plans. In a London pub, he sits with a young man named Brat Farrar, whom he spotted in the street and mistook for Simon Ashby due to an astonishing physical resemblance. Loding proposes that Brat impersonate the dead Patrick, claim the Latchetts inheritance, and split the fortune with him. Brat refuses, calling the scheme criminal, but his interest flickers when Loding mentions that Latchetts is a horse stud.
Brat is a foundling, left on the doorstep of an orphanage on St. Bartholomew's Day and given the name Brat accordingly. He drifted through France, Mexico, and the American West, discovering a deep passion for horses. He found happiness breaking horses at a ranch called the Lazy Y, where he was given a beloved grey horse named Smoky. A riding accident broke his thigh, which healed short, leaving him permanently lame. With the ranch lost to an oil strike, he sold Smoky and returned to England, where he found it impossible to get work without references. An internal voice tempts him: Latchetts could be his belonging-place. He visits Loding to see photographs of the Ashby family.
Over a fortnight in Kew Gardens, Loding coaches Brat on the Ashby family, the layout of Latchetts, and village life. Brat then presents himself to the elderly Mr. Sandal at the family solicitors, Cosset, Thring and Noble, claiming to be Patrick Ashby. He explains that his farewell note was a running-away note, not a suicide note, and that he stowed away on a ship from Westover harbour and made his way to the Americas. He deploys a coached detail, reminding Mr. Sandal of a childhood outing where Patrick had cried not from fear but because "the horses were so beautiful." The old lawyer is visibly shaken.
Bee travels to London and visits Brat unannounced with Mr. Sandal. She finds him startlingly like Simon in general impression, though the facial details differ. His quiet manner and obvious love of horses move her. The lawyers investigate for weeks, and every detail checks out. Kevin Macdermott, a barrister friend of Mr. Sandal's, remains skeptical but concedes the claimant does not "smell like a crook." A crucial identification method fails: The Ashby children's dentist was killed in the war and all his records destroyed.
The firm formally accepts Brat as Patrick. Bee breaks the news at a Sunday lunch. Simon's reaction is devastating: His face goes white, and he declares he does not believe it and never will. Eleanor is cool but reasonable. Brat agonizes over whether to go through with the scheme but cannot bring himself to flee.
Eleanor collects him at the station, and he sees Latchetts for the first time: a gabled Elizabethan house with a Queen Anne façade set in green parkland. Simon arrives and, after a tense pause, surprises everyone by declaring, "Of course you are Patrick," and shaking Brat's hand. But Brat notices something wrong. Simon's relief is not the joy of recognizing a lost brother; it is the reprieve of someone who feared the real Patrick had returned and is relieved to find a mere impostor. Simon knew instantly that Brat was not Patrick and chose to accept him for reasons of his own.
Over the following weeks, Brat settles in, studying stud books and earning the respect of Gregg the stud-groom. Simon tests him with family memories. When Brat correctly identifies "Travesty," a small bog-oak toy horse that was Patrick's treasured possession, Simon is visibly shocked, having expected to catch him out. Simon then sends Brat out on Timber, a magnificent black horse that had killed his previous owner by deliberately riding him under a low branch at speed. Simon warns only vaguely. On the ride, Timber tries to scrape Brat off against a fence rail, but Brat's reflexes save him. He realizes Simon sent him out on a known killer.
In Westover, Brat reads the inquest report on Patrick's death. The last person to see Patrick alive was Abel Tusk, a shepherd, who met him bird-watching on Tanbitches hill near the cliffs. At the village forge, the smith mentions that Simon spent the entire afternoon of Patrick's disappearance watching ironwork, giving Simon an apparently ironclad alibi.
At the Bures Agricultural Show, Simon rides Timber to a jumping victory. Before Brat's race on a horse called Chevron, he discovers that someone has loosened his girth since he left the horse with Simon. He tightens it silently and wins the race. That evening at the dance, Eleanor reaches to brush something from Brat's neck and freezes, blurting out, "You're not my brother!" She recoils in horror and flees in tears. Later, Simon, drunk and with lowered defenses, boasts to Brat that he killed Patrick. Their situation, he says, is a "wonderful spiritual twinship": Neither can expose the other without destroying himself. His motive was hatred: Patrick was "not worthy of Latchetts."
Tormented, Brat walks on Tanbitches, where he passes the old quarry, fenced and overgrown, with a sheer drop. He realizes the truth. Patrick's path from bird-watching went right past the quarry. Simon intercepted Patrick there, killed him, and threw his body in. The forge in Clare was close enough for Simon to slip away and return without breaking his alibi. Later that night, during the search, Simon planted the coat and a forged suicide note on the cliff-top. The body was never in the sea; it was in the quarry.
Brat confesses everything to the Rector, George Peck: his true identity, the impersonation, and his belief that Simon murdered Patrick. The Rector is sympathetic but skeptical. He is firm, however, that if murder was done, justice must follow.
Brat buys rope and descends into the quarry at night to search for evidence. Simon, having seen the purchase and read the label on the rope, anticipates him and waits at the edge. The two struggle at the precipice, and the rain-soaked turf gives way. Both plunge into the darkness. Simon is killed. Brat survives with a skull fracture. In the quarry, searchers find Patrick's skeleton under leaf mould, along with his treasured pen. The police surgeon concludes Patrick was not conscious when he was thrown in.
The inquest on the bones is adjourned indefinitely, the remains attributed to an unknown person. Simon's death is recorded as misadventure. Only Bee, the Rector, Eleanor, Great-uncle Charles Ashby, the lawyers, and cooperating police at the highest level know Brat is not Patrick.
Great-uncle Charles visits the hospital, takes one look at Brat, and recognizes the resemblance to his cousin Walter Ashby, a charming drifter. Investigations trace Walter to a job in Gloucestershire, where a young cook named Mary Woodward had a baby she gave up. Brat is almost certainly Walter's illegitimate son: a true Ashby by blood, though not Patrick.
Bee asks Brat to come with her to run a horse stud she has leased in Ulster, funded by Charles. Brat, in tears, accepts. Eleanor confides to Bee that she intends to marry Brat, asking only that the truth about his identity be allowed to circulate first. "Don't worry," Bee assures her. "It will get round."