Ten-year-old October lives with her father, Ezra Holt, in a hand-built wooden house deep in an English forest. They are self-sufficient: They grow vegetables, swap produce for dairy with a nearby farmer, and make rare trips to the village for supplies. October spends her days climbing trees, tending the woods, and scavenging objects from the soil, including smooth blue-green glass, which she transforms into stories she shares with Dad by the fire. Her mother left when October was four, unable to endure life off the grid. October refused to go with her, and the memory has hardened into resentment. Her mother writes letters October never reads, and when her mother visits, October hides in a tree and refuses to come down.
After a storm, October and Dad find a dead owl at the edge of the woods and bury it. Nearby, they discover a tiny baby barn owl in the sedge. Dad insists they leave it in case another owl returns, but no parent comes. Days later, October finds the baby alive and shrieking with hunger. She carries it to her secret den and names it Stig, after a character in a book about a boy living alone among rubble. Dad reluctantly helps, driving October to a pet shop for frozen mice. The breakthrough comes when a mouse October flings in frustration strikes Stig, who pounces and devours it.
Over the following weeks, Stig grows, opens her huge dark eyes, and bonds with October, following her everywhere. October considers Stig her first real friend. On her 11th birthday morning, October and Dad plant a silver birch, continuing their yearly tradition. While digging, October unearths a thin gold ring with grooves on the inside and pockets it for a story to tell Dad later.
That same morning, October's mother arrives unexpectedly, wearing a red coat and a moonstone ring. Seeing her triggers buried memories. October runs and climbs higher into a tree than she has ever gone. Dad follows, a branch splinters, and he crashes to the ground. October's mother calls an ambulance on her mobile phone, and a helicopter arrives to fly Dad to a hospital. He undergoes surgery for an axial burst fracture in his spine, a severe break requiring months of recovery.
The hospital overwhelms October. Dad is unconscious, kept asleep by medicines so his body can heal. October's mother drives her to London, to a small terraced house with cold tiles and no trees. Her mother insists Stig cannot survive in London and must go to a rescue centre. Despite October's protests, they deliver Stig to a facility run by a man named Jeff, who promises to rewild the owl for release. October's last piece of her wild world is gone.
October visits Dad daily, but he remains unconscious. She slips her blue-green glass pieces under his pillow, hoping they will work as healing stones, but nothing happens. She is consumed by guilt, convinced he fell because she ran and hid. Her mother announces October must attend school, as the law requires.
School is a sensory assault. October sits motionless and silent at her desk for four days. On the fifth day, her teacher, Mr. Bennett, sends her to the library with Yusuf, a fast-talking classmate paired with her for a school assembly project. When Yusuf carelessly drops books, October shouts at him to stop hurting them, breaking her silence. She shows Yusuf the gold ring, and he spots tiny letters engraved inside: "Let friend nor foe this secret know." Yusuf explains it is a poesy ring, a type of ring historically exchanged in pairs, each inscribed with a secret message. October becomes convinced that finding the matching ring will give her the perfect story to tell Dad, one powerful enough to win his forgiveness and bring her home.
When doctors wake Dad, October runs to him and buries her head in his chest. But Dad tells her to go home with "your mother" and says he is tired. October interprets this as rejection. On a later visit, Dad tells her that "sometimes it's a kindness to let something go even when you love it very much." October suspects he is talking not only about Stig but about her.
Her mother takes October to the Thames foreshore, where people called mudlarks scavenge the tidal riverbank for historical objects. The wide river offers October her first breathing space since leaving the woods. Her mother reveals that combing the foreshore gave her own sense of wildness in the city. October feels a spark of hope. Back home, her mother tells a story through the bathroom door about a girl who turned found objects into beautiful things. October recognizes echoes of her own life and, for the first time, puts on the pyjamas her mother bought her.
October discovers her mother is a jeweller who works in a garden shed. Her mother signs October up for the Junior Mudlarks, a supervised youth mudlarking group on the Thames foreshore, on one condition: October must keep visiting Dad.
Under the guidance of a woman named Kate, October begins officially scavenging with Yusuf. She finds coins, clay pipes, and a rusted Victorian key, but no matching ring. When Kate discovers a gold coin in the mud, October's heart leaps, but it is not the ring. She hurls the coin into the river. Kate tells her that mudlarking finds are small discarded moments that do not tell the full story of a life. October does not need a ring to fix things with Dad; he knows who she is, and he will never let her go. Not every story has a perfect ending, Kate adds, "But that bit is up to you, isn't it?"
Something inside October lightens. She reconciles with Yusuf, with whom she had fought over feeling abandoned. That afternoon, she and her mother visit the owl rescue for the last time. Stig is being released. October watches as Stig stretches her wings and swoops into the dusk. Jeff reveals he engraved Stig's name on her leg ring: Stig 2450. October whispers to her mother, "It was the right thing."
Kate helps October and Yusuf prepare their overdue assembly. They spread sand across the hall floor and hide real finds in it. October reads her story aloud: The other half of the poesy ring belongs to an owl flying free. "In the wild world flies Stig 2450." When she looks up, she sees her mother in the audience, and beside her, Dad, standing upright and smiling. October runs across the sandy floor into a circle of arms.
Dad stays in London while recovering. One evening he confesses that he never knew if raising October in the woods was the right thing, and if she is happier in London, he will understand. October tells him she wants to go home. Before they leave, she discovers her mother has painted her white bedroom into a forest: black trunks, green leaves, a starry ceiling. Her mother whispers it is for when October comes back, "if you want to." October embraces her and says "Thank you, Mum," using the word for the first time.
By October's 12th birthday, she and Dad are back in the woods. Visitors come for the first time: Yusuf and his family, her friend Daisy, Kate, and Mr. Bennett. Mum gives October a handmade gold ring set with the blue-green glass October once slipped under Dad's hospital pillow and a sliver of moonstone from Mum's own ring. Inside is engraved: "Stig 2450." October reflects that being wild and free means different things in different places, and she has a foot in each world. She sometimes hears an owl at night and thinks she sees the blink of a gold ring against the sky. There are stories everywhere, and "all the world is wild and waiting" for her.