In a future England, society is divided into two worlds separated by a physical barrier. The Conurb is a sprawling urban megalopolis where millions live in high-rise apartments, watch three-dimensional holovision, and attend bloodsport spectacles called the Games. The County is a rural, aristocratic realm where gentry live in manor houses, servants tend to their needs, and horses provide the primary transport. The two populations have almost no contact and have been conditioned to despise each other.
Thirteen-year-old Rob Randall lives in the Conurb with his father, an electrician. Rob's mother is dead, and he has carried on her habit of visiting the nearly abandoned public library. One afternoon, Rob walks to the stadium to meet his father but is caught in a violent street riot. At the stadium, his father's friend Mr. Kennealy tells him his father grabbed a live wire and has been hospitalized. That night, Rob overhears men arguing about the accident, calling it a dangerous business and implying his father had been warned. The next day, Rob learns his father died of heart failure.
With no willing relatives, Rob is to be transferred to the Barnes Boarding School, a State institution notorious for harsh discipline and bullying. He begs Mr. Kennealy to take him in, but Mr. Kennealy refuses, saying Rob will be "safer" there before correcting himself to "better looked after" (25). At Barnes, the food is inadequate, surveillance constant, and violence routine. The Master of Discipline publicly humiliates Rob and orders his library books burned. That night, senior boys beat his forehead with a rubber-headed hammer when he refuses to submit to their hazing ritual.
During warm evenings, Rob retreats to the riverbank with letters from his mother, which he found among his father's belongings. Reading them, he discovers her address was in Gloucestershire, in the County. She came from the servant class and met his father when he traveled there for work. Lying awake after the beating, Rob resolves to escape to the County, reasoning that a land of farmlands must have food.
On a Sunday, Rob walks out the main gate, catches a bus and the monorail to Reading near the County border. After narrowly escaping capture during a local Carnival festival, he heads north through derelict streets into nomansland, the uninhabited zone between the Conurb and the Barrier. He crosses the wasteland through the night and at dawn finds the Barrier: a twelve-foot fence of diamond wire mesh. He confirms it is not electrified by watching a squirrel scramble over it, then digs under a gap created by a landslip and wriggles through.
His feet blistered and near collapse, Rob is overtaken by a horseman who turns out to be Mike Gifford, a fair-haired boy about his own age and the son of a local gentry family. Rather than reporting Rob, Mike hides him in an old concrete gun battery inside a nearby hill. Over the following weeks, Mike visits daily with food, blankets, and supplies, and the two form a close friendship.
Mike's mother, Mrs. Gifford, a formidable and perceptive woman, eventually tracks the clues and discovers Rob. She decides that since he refuses to return to the Conurb, he must have a plausible identity: He will become Rob Perrott, the son of her real cousin Amanda in Nepal, sent to England following his father's death. Nepal's isolation makes the story nearly impossible to disprove. Mr. Gifford, a taciturn man devoted to bonsai (the art of cultivating miniature trees), is told the truth and agrees. Mike's younger sister Cecily is kept uninformed.
Rob moves into Gifford House, a vast gray stone manor, and Mrs. Gifford coaches him daily in etiquette. Three weeks later, he passes a critical test at a garden party. Sir Percy Gregory, the Lord Lieutenant of the County, introduces Rob to an elderly man who once lived in Nepal. The man questions Rob about Katmandu; Rob stumbles, but the man attributes his odd accent to a "Typical Nepalese settler twang" (143). Over a fine summer, Rob receives his own horse, Sonnet, and ranges the countryside with Mike. At a major archery contest, Rob finishes third, beating Mike, which earns him new respect and prompts Mike to ask detailed questions about life in the Conurb.
In September, both boys enter an elite boarding school. A senior student named Daniel Penfold holds a meeting arguing that both societies are deeply conditioned and that the ruling class must act. Afterward, Mike reveals that Penfold is connected to a real revolutionary organization led by Penfold's older brother Roger, a veteran of the China War. Mike asks Rob to join; Rob refuses, arguing the scheme is futile and that he would be sent back to the Conurb if caught. During the Christmas holidays, Mrs. Gifford privately asks Rob to watch over Mike, noting worrying school reports. Rob is torn but cannot bring himself to betray Mike's confidence.
During the spring holidays, Mike arranges an errand in Oxford while Rob goes salmon fishing at a neighboring estate. Rob sees a large troop of horsemen galloping toward Oxford and learns that a rebellion has erupted: Rebels have taken Oxford and Bristol using guns, weapons abolished from civilian life for decades. Rob rides home to find Gifford House nearly deserted, with Mr. Gifford and the male servants having joined civilian forces against the rebels. Mrs. Gifford confronts Rob, and he confesses what he knows about Mike's involvement. She accuses him of having failed to repay his debt to Mike but calls him back when he tries to leave; with the men gone, she and Cecily need him.
The revolt is quickly crushed. The government had kept a reserve of weapons, including copters that dropped nerve gas on the rebels. That night, Mike slips into Rob's room, exhausted. He tells Rob the revolt has failed but there are allies in the Conurbs; he intends to cross the Barrier and continue the resistance. Rob pleads with Mike to surrender, but Mike refuses, takes food, and rides off into the night.
Days later, Rob is taken for questioning to Sir Percy Gregory's estate. Sir Percy addresses him as "Rob Randall," revealing that the authorities have known his true identity since shortly after he moved in with the Giffords. Sir Percy describes a secret group called the guardians that manages both the County and the Conurbs, maintaining peace through psychological conditioning, channeled aggression, and concealed force. The guardians let the rebellion grow deliberately before crushing it. Sir Percy offers Rob membership, and Rob accepts. Sir Percy then asks him to help locate Mike, providing a concealed two-way radio. He assures Rob that if captured, Mike will only undergo a small, painless brain operation that will eliminate his desire to rebel while leaving him otherwise intact.
Back at Gifford House, Mrs. Gifford reveals she knows Mike visited, having noticed missing food and clothing. She says she would have turned Mike in, insisting the brain operation is harmless. Then she reveals that Mr. Gifford himself underwent the same operation as a young man; a scar hidden by his hair is the only trace.
Rob is horrified. The quiet, hobby-absorbed Mr. Gifford was once a rebel, and the operation removed the core of his independence. Rob grasps the full implications of Sir Percy's offer: He was being recruited as a puppet master over a society of surgically pacified people. He realizes Mike was right about the corruption beneath the County's surface. Rob also reinterprets his own past: The men at the Kennealys' called his father's accident a dangerous business because his father was likely part of a conspiracy, and Mr. Kennealy's insistence that Rob would be "safer" suggests his father may have been killed by the authorities.
Rob makes his decision. He rides Sonnet to the Barrier fence by moonlight, sends her home, then kneels and begins to dig under the wire, following Mike into the Conurb.