Set in a near-future Britain where an authoritarian regime outlines homes and vehicles in red paint to mark them for removal, the novel follows two siblings, Briar and Rose, through childhood displacement, brief refuge among outcasts, and eventual separation, before leaping forward to Briar's life as an adult working within the very system that destroyed their family.
The story opens with the children's mother arriving at a docking gate, the monitored departure checkpoint of a luxury art hotel in another country, to say goodbye. She is covering shifts for her ill sister Alana, wearing Alana's uniform and looking diminished. Leif, her partner and the children's non-biological guardian, drives them home in the family campervan, promising to return for their mother within a month.
When they reach home, a bright red painted line circles the perimeter of their house, stopping at the boundary with the neighbors' property. Inside, the rooms smell damp and the furniture looks like rubbish without their mother. Leif decides they must leave. They park overnight in a supermarket car park. By morning, someone has painted a red line around the campervan, and the engine will not start. While Leif argues with security, the children buy croissants, coffee, ham, and cheese inside the store. After a towtruck takes the van, they catch a train to another town. Leif announces he will travel alone to retrieve their mother, explaining that border officials always delay him because he is not biologically related to the children. He installs them in an empty former railwayman's cottage with bare plaster walls, no furniture, running cold water, and a working toilet. He leaves them forty-two tins of food, a tin opener, a kettle, and a roll of cash. Rose chants "Leif is leaving" in a singsong voice. Briar privately fears Leif may never return.
Briar calculates the tins will last seven to fourteen days and reflects on their mother's refusal to give them smartphones, her insistence that the children experience "real realities" rather than digital ones. In a flashback, their mother explains that she was a whistleblower. While working in publicity for a weedkiller conglomerate called Kindweed, she learned that the company's most popular product, marketed as safe, had the same chemical formula as their most toxic one. When she asked questions, Kindweed fired her and took her to court. She taught Briar about wildflowers and their medicinal uses, giving Briar a book with handpainted illustrations and telling Briar that their generation would make things better.
Rose wakes Briar and leads them to a nearby field where seven horses graze. Rose feeds the grey horse handfuls of grass from her open palms. Briar is struck by the horse's eye, its gentleness and what Briar will later call its equanimity. Walking to the station, Briar witnesses an elderly woman in a long black coat physically blocking a man pushing a red-paint-marking machine called a SUPERA BOUNDER. The man tackles the woman; the machine rolls toward Briar, who kicks it over, spilling paint across the pavement. The woman, Oona, an elderly activist who calls herself a "trained elusive" and knows how to evade the regime's surveillance, catches up and teaches Briar to speak without moving their lips, warning about surveillance cameras and microphones. Briar explains that their name comes from a song their mother loves.
Rose befriends Colin Kendrick, the son of a local farmer who works as a Designated Data Collector slash Strangers (DDC/S), conducting official questionnaires on behalf of the state. He wears a smartwatch "educator" that automatically films everything. Colin warns the siblings, removing his educator to do so, that facial recognition has flagged them as possible "UVs," or unverifiables: people with no official data trail. Rose names the grey horse Gliff, adapting Colin's father's description of it as "a very gliffy horse." Colin's older brother Posho describes Adult Retraining Centres (ARCs) and Child Retraining Centres (CRCs, called "circuses"), where unverifiables perform forced labor. Children in CRCs disassemble batteries with bare hands. Posho also threatens Rose with graphic sexual violence. The siblings resolve to act: Briar gives Rose their cash to buy the grey horse, and at night they free the other six horses from the field. Rose walks the horse to their empty house, where it enters through the front door.
Briar discovers a gap in the corrugated iron fencing at the dead end of their street and squeezes through into a vast overgrown space containing a former school, the School of St Saccobanda. Oona brings the children and the horse to the school, where a small community of unverifiables lives. Among them are Ulyana Yusef, a philosopher and art historian rendered stateless; a girl named Daisy; and a sullen boy called Wolf. Ulyana shows the children a reproduction of a George Stubbs painting of a white horse recoiling in terror from a lion, explaining how Stubbs induced the horse's fear artificially. Rose declares she wants "a new sublime," one where the horse breaks free, chases the stable boy, and kicks the painter out of the yard. The community takes the children in, shelters the horse during extreme heat, and gives them beds and real food. Oona sings a folk song about a rose who sustains a traveler through shipwreck and desert; Rose accepts the name Rose of Allendale. Briar bonds with Daisy, who asks whether Briar is a boy or a girl; Briar answers, "Yes I am."
Using a phone in the school's Language Lab, Briar tries to contact Leif (disconnected), Alana (no answer), and the art hotel (no record of their mother). On an old computer, Briar looks up the word "gliff" and discovers it carries dozens of meanings: a sudden fright, an inkling, a glimpse, a gleam, a substitute for any word. Briar presents the definitions to Rose, explaining she has named the horse something "polysemous," a word capable of meaning everything, anything, and nothing at once. Then Daisy disappears, lifted off the ground by strangers in a supermarket car park and driven away in a van. Oona says quietly, "It happens," and looks older.
The narrative leaps forward five years. Briar, now called Allendale, works as Day Shift Superior at a Packing Belt facility, having been captured, stripped, assaulted, "re-educated," and promoted within the authoritarian system. Briar describes the "voids," unmonitored basement spaces where those in power commit acts of violence without record, stating: "One line about it is more than enough" (226). A worker named Ayesha Falcon, roughly seventeen with severe injuries from dismantling batteries barehanded in a CRC (one hand a blunt stub), is sent to Briar's office. She says Briar is "the image of" their sister. Briar denies having a sister, having spent years scrubbing Rose from all data systems. But Ayesha uses the word "gliff," explaining that Rose used it as the group's name for painkillers. Rose never took anything herself and told them that anyone who looked like her would help.
In an unmonitored void, Ayesha describes meeting Rose in a northern cave with ancient wall carvings. There, Rose told the group a story: A tyrant kills his opponent, burns the body, and locks the ashes in vaults, but the tyrant hears the ash crumbling everywhere. He flushes the ashes, launches the container into space, yet realizes his opponent has become the water, the air, the universe itself, the opposite of destroyed. Briar acts. Briar disables all cameras, reassigns every Packing Belt worker to a better sector known as a recruitment site for the Campion (an underground resistance network), and deletes all technical data, ARC and CRC records, and personal records from three systems. "Now that I don't exist I finally exist again" (260). Briar walks out, leaves all devices and identification on a bridge as if someone has jumped, shouts "Colon! Campervan!" at a surveillance camera (code words Rose once coined for imagined technology that would destroy stored data and end the surveillance of travelers), and disappears heading north.
The final section shifts to second-person narration addressed to Rose. Eleven years old, riding the horse after Briar's capture, Rose returns to the demolished house and finds the glass top from their mother's perfume bottle in the neighbors' dustbin, still carrying her scent. She shows the neighbors a stone, chooses not to throw it, mounts the horse, and rides away into a rainstorm. Colin catches up, clay-stained. He says he wants to help. Rose takes the wet, blurred piece of paper with the definitions of "gliff" from her pocket and uses it to wipe the clay from his face. She starts walking. The horse follows. Colin follows. Rose says over her shoulder: "We'll be making it up as we go" (277).