On a ranch in the mountains, Derrek Edlund prepares to ride out to find his father Grady, who has been missing for 10 days along with two ranch hands, Chuck, Billy, and Jasper. As his younger brother Tim looks on, their uncle, Rowdy, advises against it because of the weather and reluctantly agrees to do it himself despite a bad leg. Before leaving, he tells his sister-in-law, Elena, that he's leaving and warns that Grady doesn't take the ranch seriously. They're interrupted when the horses whinny and they look outside to see Grady staggering in from the woods. Rowdy and the other ranch hand, Eddie Bear, bring him in but he's in bad shape.
The family puts Grady to bed and calls in the doctor to examine him for what appears to be frostbite. Tim is worried that his father is going to die and Derrek tries to reassure him. The doctor says that Grady will be all right but wonders how he managed to stay fed for a week and what happened to the other three men. As he leaves, he notes that Grady did say one thing: that he's hungry.
Derrek tends to the horses and Rowdy tries to give him advice. Derrek is angry that his father had to buy the ranch from Grady to keep it going, and accuses him of having an affair with his mother Helena. Derrek slugs Rowdy, who shrugs it off and tells him to help his mother. Derrek accuses him of wanting Grady dead, but Grady warns that Derrek doesn't knowing everything he thinks he does. They go inside and Derrek takes a meal up to his father. Once he's gone, Helena admits to Rowdy that she thinks that Grady looks like someone else. They hear a noise upstairs and go to find that Grady has shoved away the food. As Helena makes her husband comfortable, Grady licks her arm and whispers that she "tastes good."
That night, Rowdy is dozing in the main room when the horses start whinnying again. He goes out and finds Eddie Bear, and they discover that one of the mares has been killed by a wild animal. Upstairs, Grady watches them from the bedroom window.
The next day, Rowdy tries to get Sheriff Gordon to come out without success. Eddie Bear reports he hasn't found anything but wants to talk to Grady, who he thinks already knows about the dead mare. Helena is watching over her husband, and Eddie Bear tells them to wait downstairs. Eddie draws a knife and talks about his uncle disappeared when Eddie was a boy. The uncle came back and looked like Grady, and Eddie says that his uncle's family was torn apart. Eddie tells Grady to let go whatever is eating at him. Grady lurches up and tells him it's already eaten him up, and to put away the knife. He warns that nobody will believe him if he kills Grady. Eddie reluctantly puts away the knife and leaves.
Later, Eddie tries to explain to Rowdy what happened, and that Grady is stronger than they think. He tells the legend of the Wendigo, a hateful spirit of the lonely places that lives in the high mountains. It gets inside people when they're weak and starving. Eddie's uncle felt his people were wronged, and when he got lost the Wendigo took control of his spirit. When the uncle came back, livestock and people went missing. The sheriff gunned down the uncle and Eddie left the reservation for good. Eddie warns that whatever they call it, the Wendigo is a creature of hate.
Helena goes to see Grady who claims to have amnesia. Helena tries to get an explanation from him and he talks about he and the others got lost and holed up in a cave to stay warm. It snowed and Chuck and Billy went for help. He and Jasper started a fire and then Grady heard a voice asking to be let in. It then told Grady that he had to "eat," and Jasper was too weak. Helena thinks he's delirious and backs away, but he comes after her and talks about the kids. He grabs her by the throat but Rowdy arrives with a gun and tells him to let Helena go. Grady releases her and howls in anger… then runs outside before Grady can shoot him. Rowdy and Helena get the boys out to the front door, where Eddie comes in with an axe.
Grady rampages upstairs while Eddie tells Helena to get the boys to his trailer. Helena tries to explain to Derrek that Grady isn't Grady anymore. The howls stop and Eddie and Rowdy try to flank the possessed Grady. Eddie goes upstairs but Grady ambushes him.
Rowdy goes to the trailer to tell them that Grady and Eddie have both disappeared. Rowdy gives a gun to Derrek but the two sons don't want to see their father hurt and think he's just sick. Rowdy goes to get another gun but tells Helena to tell the boys the truth when the time is right. He goes to the gun cabinet in the house, arms himself, locks the cabinet behind him, and goes upstairs. Rowdy finds a dying Eddie, who passes away. He finds Grady in bed, who claims he's too sick to get up. Grady says that he couldn't help himself, and reveals that he's getting even thinner. He begs for help but when Rowdy says he doesn't know how, Grady accuses him of wanting to get rid of him so he can get the ranch and Helena. Rowdy considers a picture of himself and Grady as boys, then aims the rifle. Grady lunges at him…
At the trailer, Helena hears screams and howling noises, and says they have to get out. They head for the barn with the keys and Helena has them hide while she takes the gun. The possessed Grady enters the barn and asks her to put the gun down. She reluctantly lowers it and he takes it away from her. He then pulls Rowdy's corpse around the corner and takes Helena to the kitchen where he forces her to start preparing a stew. She tries to get through to Grady within, but the Wendigo tells her to cut Rowdy up. She refuses and he slaps her to the ground, where she sees Derrek hiding in the house. She agrees and takes the cleaver and cuts up her brother-in-law's body. Meanwhile, Derrek gets the key to the gun cabinet and tries to get it open.
The Wendigo tells Helena to prepare two bowls and forces her to eat it. She snaps at him to keep him from seeing Derrek and they sit at the table. However, Derrek drops a shell and Helena hastily starts eating to distract him further. The Wendigo talks about how Grady felt the rage growing inside him up in the mountains as he thought of Helena and Rowdy. The Wendigo gobbles down the stew then says that Rowdy and Helena did it to him. She hits him with a bowl and Derrek comes in and shoots him. Grady slams Helena into a cabinet and runs outside, wounded. Helena has cut her wrist on the cabinet glass and Derrek tends to her wound.
The Wendigo goes to the barn and calls out for Tim. He finds the boy and catches him, but Derrek arrives and tells it to put Tim down. When Derrek calls him "Daddy," the Wendigo says that he isn't Derrek's daddy. Tim bites off his ear and the Wendigo drops him, giving Derrek a clear shot. The two brothers get outside and Derrek explains that Rowdy isn't their uncle, but their father. However, the Wendigo is still alive and grabs Derrek, but Helena arrives, picks up the gun, and kills her possessed husband.