Mattie lives in a remote mountain cabin with William, a powerfully built older man she believes to be her husband. William controls every aspect of her existence: He dictates her movements, forbids her from straying far, and beats her for any perceived disobedience. She has no contact with the outside world and is only permitted to read the Bible. William keeps a locked trunk she is never allowed to open, with the keys always on his person. Each night he forces himself on her, insisting a man must have sons. Mattie has endured two miscarriages and one stillbirth; after the second miscarriage, William beat her so severely he permanently damaged her left arm.
While checking rabbit snares, Mattie discovers a dead fox, killed but uneaten, with viscera scattered across the snow. Beside it is an enormous paw print, roughly twice the size of a grizzly's, with unusually long claw marks. Only rear-paw prints are visible, as though the animal walks upright. She reports to William, who strikes her and inspects the scene. They follow the creature's trail into a clearing where the prints inexplicably vanish, and Mattie spots claw marks gouged high on a tree trunk. An unearthly cry fills the forest, a composite of bear, mountain lion, eagle, and something almost human. Mattie has fleeting memories of a girl named Heather, someone from her lost childhood later revealed to be her sister, but William insists these memories are dreams she invented.
William insists Mattie accompany him to track the creature to caves on a cliff face above a meadow. Inside one cave they find bones from dozens of species sorted into organized piles alongside organs in varying stages of decay. As they exit, a young man named Griffin Banerjee, an amateur cryptozoologist investigating reports of the unknown creature, appears in the meadow below, photographing tracks. William confronts him aggressively and raises his rifle when Griffin mistakes Mattie for William's daughter. Mattie talks him down by arguing that a missing hiker would bring search parties to the mountain. On the walk home, they find animal corpses hanging from branches by their own viscera. The sight triggers a Christmas memory: stockings on a mantel, one reading HEATHER and the other SAMANTHA. She whispers her true name for the first time: "Samantha. Not Martha. Samantha" (51). William beats her unconscious.
William abandons the injured Mattie in the woods. She hauls herself from tree to tree, her left eye swollen shut. At the stream the creature blocks her path, forcing her back toward the cabin, where she finds the door bolted from inside. William is awake but refuses to let her in. She hides in the outhouse; the creature circles outside but does not attack. At dawn she finds symbols scratched into the snow by the creature's claws, which she interprets as a warning to stay away from its lair. William chose his pride over her life, and for the first time Mattie resolves to escape.
William leaves for town, convinced the creature is a demon. Alone, Mattie has fragmentary memories: William at a table across from a young Samantha, her mother in a yellow sweater standing behind him. She realizes William was part of her childhood, not merely a husband she cannot remember marrying. While William is away, Griffin and his friend C.P. Chang arrive at the cabin. They are amateur cryptozoologists, people who search for supposedly mythical creatures like Sasquatch. Mattie warns them about the creature through a window, but they are fascinated rather than frightened. William returns with weapons: a massive rifle, a steel bear trap, grenades, and poison. While doing laundry, Mattie finds a roll of hundred-dollar bills in his pocket and hides it.
Mattie later discovers William confronting Griffin, C.P., and their friend Jen in the woods. William strikes Griffin with a shovel, then seizes Mattie by the throat when she intervenes. In his fury he reveals the truth: "I should have killed you then like I killed your mother" (148). Griffin and Jen fight back, and the four flee, C.P. carrying Mattie. Griffin recognizes her from a televised age-progressed photo: She is Samantha Hunter, a kidnapping victim whose case has been in the news for 12 years.
The escape is agonizing. Griffin is barely conscious, and Mattie can hardly walk. A horrifying scream suggests the creature attacked William. As they rest by boulders, Mattie spots two sets of eyes watching from the trees above. Before she can react, the creature swoops down and snatches the unconscious Griffin into the canopy. Mattie insists they return to the cabin for weapons. On the deer path, Jen steps into William's bear trap. C.P. frees her, but she loses consciousness, likely poisoned by a substance William applied to the trap. They hear William's limping approach and rush to the cabin, where Mattie bolts the door.
William demands entry. Mattie refuses despite every instinct screaming at her to obey. The creature attacks outside; gunfire and Griffin's screams fill the night before abrupt silence. The creature smashes the window and deposits a still-warm human heart on the worktable. They barricade themselves inside and tend to Jen. C.P. makes grilled cheese sandwiches, the first food anyone has prepared for Mattie in 12 years. As she sleeps, Mattie relives the kidnapping: Eight-year-old Samantha let William in through her bedroom window. He killed her mother, carried Samantha out, and locked her in a wooden trunk in his truck.
C.P. finds William's keys in the snow, including one for a vehicle. Mattie unlocks the trunk and finds packets of heroin, stacks of cash, and newspaper clippings about her own disappearance and her mother's murder. She reveals the Box, a coffin-like space beneath the cabin floor where William confined her as a child to break her will. She cuts off her waist-length braid with a kitchen knife: "William liked my hair long" (283). They load Jen onto a sled and leave.
As they travel through the woods, the creature drops from the trees and takes Jen, vanishing into the canopy. Mattie drags the stunned C.P. to the open ground by the stream. C.P. insists on retrieving Griffin's camera and notebook from their stashed packs to honor his friend. They discover both Griffin's and Jen's bodies hanging from trees by their own intestines, their torsos hollowed out, mirroring the animal displays near the creature's cave.
William, hideously wounded but alive, ambushes them at the packs. He shoots C.P. in the shoulder and advances on Mattie, insisting she is his property. She declares her true name: "I'm not Martha. I'm Samantha" (306). Darting to his blind side, where the creature destroyed his right eye, she pushes him off a cliff to his death.
Mattie and C.P. follow the stream to a dirt road. The creature bursts from the trees, severs C.P.'s hand along with the rifle, and retreats. Mattie binds the stump and sees the creature clearly for the first time: a smaller one clinging to the back of a larger one, a parent protecting its young. This explains its relentless aggression. Mattie tells the creatures they are leaving and not coming back. They find William's truck, drive down the mountain, and pull into a state troopers' barracks, where C.P. passes out from blood loss. Mattie walks inside alone. When the officer asks her name, she answers: "Samantha. My name is Samantha" (320).