A Boy and His Dog at the End of the World

The world has undergone a "soft apocalypse" called the Gelding, an unexplained event that rendered nearly all of humanity infertile. Over a single lifetime, the global population dwindled as the Lastborn generation aged and died without producing children. Only rare outliers, about one in a million, retained the ability to reproduce. Griz, the narrator, lives on Mingulay, a small island off the Atlantic coast of Scotland, with father Abraham (Abe), a mother who no longer speaks due to a head injury, older siblings Ferg and Bar, and two mongrel terrier dogs, Jip and Jess. Jess is an extremely rare breeding female, since whatever caused the Gelding also affected dog fertility. Griz's sister Joy supposedly died years earlier after falling from a cliff; their mother's head injury occurred when she rushed down the slope in grief. The family's nearest neighbors, the Lewismen, live five islands to the north. Griz addresses the narrative to a long-dead boy in a found photograph, using the image as an imaginary audience for a diary.

A stranger named Brand arrives at Mingulay under bright red sails, claiming the Lewismen directed him. He offers a needed windmill part, pre-Gelding liquor, and homemade marmalade, sharing tales of 13 years wandering, including time in Spain, the Stockholm Archipelago, and the North Sea. He shows keen interest in the dogs, particularly Jess. That evening Brand spreads the marmalade on bread for the family. Griz alone avoids eating it because a chipped tooth makes the sugar painful.

The next morning the family lies drugged and vomiting; the marmalade was laced with a sedative. Brand has stolen Jess, the dried fish stores, and Abe's coat. Without waiting for the family to recover, Griz grabs supplies, Ferg's gun, and a bow, then sets off in pursuit aboard a small yacht called the Sweet hope, with Jip.

The chase leads south through unfamiliar waters. At nightfall Griz paddles a kayak to Brand's moored boat, but Jess is not aboard. Griz steals Brand's navigation chart and sabotages his anchor. On Iona, Griz is knocked unconscious by Saga, Brand's large dog, alive despite Brand's earlier claim of her drowning. Brand ties Griz up, reveals Jess is locked in a shed chained to Saga, and refuses to return her. He warns Griz never to follow and sails away, having cut the Sweet hope's anchor ropes to set it adrift.

Griz repairs the damaged kayak paddle and chases the drifting yacht through a squall. The boat has snagged on shipping containers tangled in fishing net. Griz spends hours cutting through the netting, sustaining deep cuts and shellfish scrapes on one arm. Once free, Griz sails southeast toward the mainland. Following marks on Brand's chart, Griz deduces that Norfolk is Brand's home base. At Blackpool, a ruined seaside town, Griz climbs a tall tower and sees the Sweet hope burning at the jetty below. Brand has torched the boat and scratched a warning in the sand telling Griz to go home. That night, Griz spots a faint light on the horizon and takes a compass bearing, hoping it signals other people. With no vessel, Griz resolves to walk to Norfolk, steal Jess, and take Brand's boat.

The journey east is grueling. Griz navigates by compass through overgrown countryside, encountering trees and wildlife for the first time. The arm wounds fester, and Griz collapses with fever for days. After recovering, Griz wakes one morning to find Jip gone. Days of searching yield nothing. The compass bearing leads to a ridge where a lightning strike ignited a brush fire; there are no people. Overcome with grief and dashed hope, Griz presses on alone.

A wounded boar charges Griz. A field guide in a front pocket deflects the boar's tusk from the femoral artery. Griz escapes into a pile of rusted cars until a mounted woman shoots the boar dead. The woman is John Dark, a grey-haired Frenchwoman travelling with three horses. She has caught Jip alive in a snare. They communicate through mime and a French-English dictionary. John Dark crossed from France through an undersea tunnel on horseback. Her daughters died of a plague brought by a "Freeman," a member of a group that maintained underground computer installations. She has been hunting the Freeman across the mainland for vengeance.

When John Dark spots a Freeman pendant Griz found atop the Blackpool tower, she attacks, believing Griz is connected to her daughters' killer. After they survive a wolf attack together that night, trust is restored. Riding east, they discover the Homely House, a preserved stone home left open by its former owners, whose skeletons lie intertwined in an upstairs bathtub. They rest, eat peaches from a walled garden, and play music on a wind-up gramophone.

John Dark's horse breaks through the corroded roof of a buried water tank. John Dark suffers a broken leg, broken ribs, and a head wound. Griz kills the horse, splints the leg, and retreats to the Homely House. After weeks of nursing, John Dark insists Griz continue alone. Her parting note reveals she has seen through Griz's disguise as a boy, comparing Griz to her own daughters. That she tells Griz to take both horses signals she does not expect to survive.

Griz rides to the Norfolk coast and spots Brand's red sails at a riverside settlement. Scouting alone, Griz is captured by three masked riders who call themselves Conservators, a group dedicated to repopulating the world. Because a previous Freeman visitor brought a deadly plague, all outsiders must undergo a month of quarantine. Griz is imprisoned in a bunker where Brand is already held.

Brand explains that the Conservators keep women and girls in a fenced compound as "breeders" and have historically stolen or traded for girls from other families. Because nightly inspections require prisoners to strip, Griz reveals to Brand what the Conservators will discover: Griz is female, disguised as a boy since childhood at Abe's insistence as a protective measure.

A Conservator woman named Tertia removes her mask to announce Griz is female. Griz recognizes Tertia as Joy, the sister long believed dead. Joy punches Griz through the bars, accusing the family of having sold her to the Conservators. Ellis, the group's patriarch, orders Griz's cell door permanently locked. Brand reveals that Ellis raped Joy as a child, resulting in a stillborn baby. When Ellis later attempted to assault Joy again, she scarred both herself and Ellis with a red-hot poker.

Trapped, Griz uses a hidden multitool to scrape mortar from a block in the cell wall while writing the journey's full account in a notebook. The notebook vanishes from a window ledge; two days later, Joy appears, having read it and learned the truth: The family never sold her; she was stolen. Joy provides tools to push the loosened block free, unlocks the gate, and reveals she has poisoned Ellis with herbs Bar once taught her. They leave Brand locked behind the gate, his boat disabled but not destroyed. Joy warns Brand never to follow but suggests he bring his sisters, implying Ferg might like to meet them.

Griz and Joy ride north with the dogs and horses. At the Homely House, John Dark is alive and joins them. In Glasgow, they camp in a great library, where Griz discovers a Freeman text revealing the group's failed attempt to preserve human consciousness in computers. They repair a boat and sail home to Mingulay. The final scene shows Griz writing the last pages by the fire, surrounded by Joy, John Dark, Bar, their mother, and the dogs, reflecting on identity, imperfection, and being "surprised by joy," at once the sister, the emotion, and the poem.