Riley, a teenager living in Boulder, Colorado, steals milk from a grocery store to feed her seven-year-old brother Oliver. Their guardian, a man they call Cousin, starves the children as part of a ritual he claims will purge a demon from them. He forces Oliver to carry a box of bricks up and down stairs daily and subjects both children to invasive inspections. Riley endures this while planning to wait until she graduates before taking Oliver and running.
One evening, a girl named Noon taps at Riley's second-story window. Noon lives with other runaway children at a place in the Rocky Mountains called Nowhere, the former estate of Leaf Winham, a famous movie star who murdered young men there before killing himself during a fire that destroyed his house. Noon gives Riley handwritten directions. Riley refuses but keeps them.
Riley decides they can no longer wait. She poisons Cousin with rat poison baked into his strawberry pie, retrieves her dead mother's silver locket, takes Cousin's .22 pistol, and wakes Oliver at midnight. She manipulates Oliver into fleeing by telling him the demon is coming, using Cousin's own language against him. They catch a bus to the edge of Rocky Mountain National Park and enter the wilderness on foot.
On their second night, a man approaches their fire. He has a missing little finger and moves toward Riley with predatory intent. She shoots him twice in the eyes. A third bullet strikes Oliver in the calf. At dawn, Riley discovers the dead person was a boy not much older than her, and what she mistook for a knife was a compass. She rolls the body downhill, treats Oliver's wound, and presses on.
Lost above the snowline on the third day, with Oliver feverish, Riley collapses. Two Nowhere children find them: Cal, a boy with straw-colored hair and a missing little finger, and Everett, a silent figure in a black balaclava. They carry Riley and Oliver along a cliff path and down a zipline into a hidden valley. Riley sees the burnt-out ruins of Nowhere House and notices Cal and Everett averting their eyes from it, breathing into their palms and releasing their breath toward it in a ritual gesture.
Noon stitches Oliver's leg. They are placed in a stable stall belonging to Cal's brother Danny, who is away. Noon explains three rules: avoid Nowhere House, never say Leaf Winham's name, and protect the children. Five young children live at Nowhere alongside a crocodile named Tinkerbell, a survivor from Winham's private zoo that inhabits the valley's lake.
Riley witnesses the sunset worship ritual and realizes the group reveres the place in Winham's name. She tries to flee with Oliver but finds the zipline gear removed. Noon explains that worship keeps the community unified. A mountain lion prevents their departure, and Riley accepts they will stay.
A parallel narrative introduces Marc, a documentary filmmaker with a prosthetic leg, and his camerawoman Kimble. They investigate Annie Lyons's claim that the Nowhere children kidnapped her, bled her daily, and fed her mushrooms. Kimble finds a contact in Linus, a hardware store worker whose throat bears scars hidden by a rose tattoo. Years earlier, as a young firefighter, Linus responded alone to the fire at Nowhere House. He found Adam Leahy, an architect who had built a secret staircase for Winham, propped against a tree with his throat already slit. Winham appeared, broke Adam's neck, then poisoned and slashed Linus. Linus survived only because arriving fire engines prompted Winham to take his own life. Investigators later found a hidden room containing photographs of boys.
Another timeline tells Adam's story more fully. Adam arrived at Nowhere to build a staircase with peepholes into every room and fell in love with Winham, staying for months. When Adam discovered the hidden room and its evidence of serial murder, he set fire to the staircase and escaped through a tunnel beneath the foundations, past a lilac tree and five smooth round stones in the earth. Winham caught him outside, leading to the encounter Linus witnessed.
Back at Nowhere, Riley carries a crushing secret: The boy she killed on the mountain was Danny, Cal's brother, recognizable by the matching missing finger. She lies and threatens Oliver into silence. When a blight strikes, Noon declares they must perform a ritual called "blood in the land." They raid the nearby town of Ault for supplies, then kidnap a local woman named Alison after watching her drug her own children. Inside Nowhere House, they cut Alison's arm above a sunken garden so her blood drips onto earth where five smooth round stones surround a lilac tree. Riley joins the chant. Rain returns the next morning.
Riley's secret unravels when Cal finds Danny's remains on the mountain, with bullet holes in the skull. Oliver, questioned by Noon, tells the truth. The group votes to give Riley to the house. They strap her into the bloodletting chair, drug her with hallucinogenic mushrooms, and cut her wrists. Noon smashes Riley's locket open, revealing that Riley's father was Adam Leahy, killed by Winham three days before Riley was born.
A historical chapter reveals that long before Winham, Nowhere was an apple farm where Thomas Dunning murdered his wife and five secretly kept children. The townspeople buried the children in the cellar and covered up the killings. Riley comes to believe these murdered children are the same figures she knows as Hallie, Rufus, Whitey, Peach, and Una.
Riley gnaws through the leather restraint, retrieves Oliver, and sets the stables ablaze to cover their escape. They flee through the tunnel beneath Nowhere House. In its depths, Riley gives Oliver a compass and tells him to forget everything, including her name. She insists she must stay to care for the children and sends him away.
A chapter titled "The Lilac Boy" describes Big Bobby Sullivan, a local Ault resident, finding a feral child in a lilac tree on the mountain, saying only "Dead. All dead." The boy was never identified and was adopted by a family far away. This child is Oliver.
Marc's true purpose is not the documentary. His daughter Silvie has Berger's disease, a kidney condition requiring a third transplant. Both Marc and his ex-partner Claude, Silvie's other parent, have already donated kidneys. Marc is Oliver, the Lilac Boy. After Riley sent him through the tunnel, he was hospitalized, his infected leg amputated below the knee. He was adopted and renamed Marc Villeneuve. The documentary is his cover for finding Riley.
During a catastrophic storm, the gate to Nowhere collapses. Marc enters, finds Riley injured in the house, and identifies himself by repeating a childhood joke. Riley embraces him. Marc asks her to donate a kidney. Riley insists her ghost children need her. Marc argues that the children died in the fire she set, that they were real children locked in the stables. He persuades her: "Your children would want you to help" (280).
They escape through the flooding tunnel as Nowhere House collapses. The crocodile attacks Marc, seizing his prosthetic leg. Riley shoots it in the eye, saving him. Riley consents to the donation. The transplant succeeds, and Riley bonds with Silvie in the hospital. But the next night, Riley vanishes from a locked fourth-floor ward through an open window. Marc does not wonder where she went.
Three years later, Marc lives in the Arizona desert with Silvie, who is healthy. He reflects that Riley is a question he will never stop asking. Maybe she is nowhere. Maybe she is somewhere she belongs.