Dylan Hickock, a 14-year-old boy in Portland, Oregon, narrates events that began when his father, Bill Hickock, a real estate agent with no outdoor experience, returned from a hunting trip on Mount Saint Helens looking battered and exhausted. He asked only, "How late is the library open?" before falling asleep. For four months he behaved with suspicious normalcy while Dylan was occupied with school and the swim team. Dylan's mother, a Ph.D. student in archaeology at Portland State University, is preparing for a five-month dig in Egypt, the final requirement for her doctorate.
On the day his mother departs, his father spots a startling headline at the airport and drags Dylan to a meeting of Bigfoot International (B.F.I.), a Sasquatch enthusiast organization. Clyde Smithers, a longtime B.F.I. member, presents slides showing a large apelike creature carrying a deer up a steep slope. Dylan's father weeps silently during the presentation. Dylan connects the slides to the hunting trip and realizes his father believes he saw a Sasquatch.
At his father's real estate office, Dylan discovers it has been transformed into a research headquarters. His father recounts his encounter: He saw a giant apelike creature, fell backward in terror, and awoke in a shelter made of fresh fir boughs 200 yards away, with no idea who moved him. He concealed his research so Dylan's mother would not cancel her trip. That night, Dylan researches Sasquatch online, learning about cryptids, creatures reported to exist without scientific proof, and the long history of sightings in the Pacific Northwest.
At a second B.F.I. meeting, Dr. Theodore Flagg, a cryptozoologist who studies animals rumored to exist but not yet scientifically proven, announces a funded expedition to capture a Sasquatch dead or alive. An old man passionately objects, calling the plan murder. Dylan's father identifies him as Buckley Johnson, nicknamed Buck, a longtime tenant and former biologist for the U.S. Fish and Wildlife Service. At Buck's house, a life-size wooden Sasquatch carving in the backyard gives Dylan his first sense of the creature's scale. Across the street, a rental house has been converted into a surveillance post with cameras aimed at Buck's door. Over dinner, Buck calls Flagg a fool and advises Dylan's father to forget the Sasquatch. When asked if he has seen one, Buck answers only, "In my dreams."
After three days of deliberation, Dylan's father joins the expedition, which moves into the Hickock home under strict security. He explains privately that he joined to sabotage the effort from within, inspired by Buck's suggestion that a Sasquatch may have built the fir-bough shelter where he awoke after his fall. Flagg imposes rigid rules: The team lives together with no private phone calls, and no one leaves alone. A professional tracker hired by Flagg for the expedition, Kurt Skipp, arrives: a quiet, young man who demonstrates alarming knife skills. Dylan's father hides a radio and map for delivery to Buck, and the team departs. Dylan resolves not to stay at his friend's house as instructed.
Dylan forces Buck to take him along by threatening to go to the mountain alone. They drive up just ahead of barricade crews establishing restricted zones around the volcano. Buck leads Dylan to a century-old log cabin containing a padlocked storage locker he calls "Pandora's box." That night, a terrifying roar jolts Dylan awake. The next morning, he discovers massive footprints in the volcanic ash, over 20 inches long. Buck admits the sound was a Sasquatch and confesses he has seen them many times, estimating a dozen live in the area. He tells how the cabin's previous owner, Billy Taylor, a hermit who lived in the woods for 50 years, once communicated with a Sasquatch using hand signs, calling it "a neighbor." Billy left Buck the cabin, and over years the creatures accepted his presence.
During days of rain, they monitor the expedition by radio. The teams find nothing, but Clyde reports branches snapping in a ravine, which alarms Buck. Joe West, the B.F.I. chapter president, argues the search is foolish near an active volcano. When the sun emerges, Dylan encounters not a Sasquatch on the trail but Agent Steven Crow, the retired F.B.I. agent who had been operating the surveillance post across from Buck's house. Crow reveals that Buck hijacked Northwest Airlines Flight 305 on November 24, 1971, under the alias Dan Cooper: He boarded with a fake bomb, demanded $200,000 and four parachutes, released the passengers in Seattle, then jumped from the airplane over Mount Saint Helens. The case became famous under the erroneously reported name D. B. Cooper. Dylan returns to the cabin to find Buck gone.
Dylan finds the locker's padlocks hanging loose. Inside, Buck's walking staff unscrews to reveal a key that unlocks a trapdoor. A ladder descends 200 feet into a cavern littered with bones and antlers: the Sasquatch's underground lair, connected by lava tubes, tunnels formed when surface lava cooled over still-flowing molten rock. Dylan enters the tubes searching for Buck. An earthquake strikes, and he wanders lost until he stumbles into a cavern containing the hijacking ransom, stashed by Billy Taylor. Buck appears with a lantern, guided by a Sasquatch that tracked Dylan's shouts. Buck confesses his son Gary was dying of cancer, and he hijacked the plane in desperation. A female Sasquatch found him after landing and carried him to Billy's cabin, just as one later carried Dylan's injured father to the fir-bough shelter.
Buck shows Dylan rubber footprint molds used to create false trails diverting searchers from the tube entrances. The plan is for Dylan to stamp fake prints leading away from the ravine, where a hidden entrance behind a waterfall opens into the tubes. Over the radio, they learn Joe and two other team members have quit, but Flagg persuades Clyde to search the ravine one final morning. Agent Crow's voice breaks through: He has crashed his car and his leg may be broken. Feeling responsible, Buck insists on rescuing Crow, leaving Dylan to execute the plan alone.
Dylan emerges behind the waterfall and begins stamping fake tracks. Kurt Skipp steps from the darkness with a rifle, having traced coded radio conversations to the ravine. He forces Dylan upstream. A real Sasquatch appears 50 feet away. Kurt raises his rifle. Dylan charges into him; the gun fires as they fall. A bright flash fills the sky: Mount Saint Helens erupts.
The explosion throws Dylan into a beaver pond, where he survives by diving underwater and using his coat to filter searing ash. Kurt is killed by a falling tree. Dylan's father calls weakly over the radio, injured near the waterfall. Dylan finds him with ash burns and damaged lungs, drags him through the passage to Lava Lake, and tends a fire beside him. As Dylan prepares to hike for help, a Sasquatch wades across the lake, gently touches his father's bandaged forehead, lifts him, and carries him down the mountain road. When a rescue helicopter appears, the Sasquatch sets his father on the road and vanishes. Dylan tells the crew there were only two people.
In the hospital, his father asks first whether the Sasquatch are safe. Dr. Flagg, treated for a broken arm, announces plans to write a book but never visits. Clyde and his brothers are never found. Buck's Jeep is discovered empty on a logging road. Then an email from Buck directs Dylan to expect a visitor: Agent Crow arrives on crutches, alive. Buck rescued him before the eruption, and someone pried open their buried car, which Dylan infers was a Sasquatch. In Dylan's garage freezer, Crow finds the ransom money, Buck's walking staff, and two envelopes. Crow's envelope contains Buck's signed confession to hijacking Flight 305 under the alias D. B. Cooper. Dylan's contains the deed to Buck's cabin and a note saying Lava Lake has been "closed down" for the Sasquatch's safety. Dylan examines the walking staff and finds a newly carved face: one that looks like him. The Sasquatch's secret endures.