The film opens with the capture of a girl in the woods, who is taken to a cabin with cult members. Her father immolates her, as it is revealed she is not human but actually a demon.
After a long time, David and his girlfriend Natalie arrive at a cabin in the woods, where they meet with his younger sister Mia and his friends Eric and Olivia. The group plans to stay in the cabin while Mia overcomes her addiction to heroin. The cabin’s cellar—the same one in which the girl was immolated—is littered with rotting animal corpses, a shotgun, and a book called the Naturom Demonto.
Eric, despite written warnings, reads aloud an incantation and awakens a malevolent force. Mia begins seeing a bloody girl in the woods, and begs the group to leave. They refuse, believing that she is experiencing withdrawal. Mia takes Eric’s car and leaves the cabin. Mia, while driving, sees a naked girl and swerves the car out of the way, crashing into a tree. Mia then exits the car and runs into the woods. While in the woods, several vines from a demonic tree grab Mia’s arms and legs, preventing her from escaping. Mia sees an Abomination version of herself. A vine comes out of its mouth and rapes Mia, successfully possessing her.
David finds his murdered dog along with a bloodied hammer, and goes to confront Mia, who is in the shower. He sees her scalding herself in the water and tries to drive her to a hospital but heavy rains flood the road. That night, the possessed Mia shoots David in the arm and warns that they will all die. David locks her in the cellar but not before she vomits all over Olivia. Olivia becomes possessed and stabs Eric in the chest; he bludgeons her to death. David nurses Eric’s wounds as Eric explains that when he read the book it released “something evil”. Mia lures Natalie into the cellar, where she bites her hand. Mia sexually molests Natalie’s thigh before planting a bloodied kiss on her mouth. David opens the trapdoor, allowing Natalie to escape. The demon tells him that Mia no longer exists.
Eric explains that according to the Naturom Demonto, the Taker of Souls must claim five souls in order to unleash the Abomination. Natalie becomes convinced that her arm is infected and amputates it with an electric knife. David patches up her wounded arm while Eric explains that Mia must be “purified” either by live burial, bodily dismemberment, or burning. The possessed Natalie attacks the pair with a nail gun but David shoots her. Natalie turns back to normal but bleeds to death in David’s arms.
David incinerates Olivia’s corpse, dismembers Natalie’s body, and plans to burn down the cabin with Mia in it. However, as Mia starts singing a song from their childhood, he has a change of heart and decides to bury her instead. He digs a grave and tries to subdue Mia, who attempts to drown him. Eric intervenes but not before being fatally stabbed. David sedates and buries Mia as the demon possessing her taunts him. He then unearths and defibrillates her, begging her to return to him. The demon is exorcized and Mia is healed as the siblings reconcile. David enters the cabin to retrieve the car keys, but the vindictive demon returns by possessing the body of the deceased Eric and stabs him with barbed wire cutters. As Mia tries to help him, David locks her out to protect her and shoots a gasoline can, destroying Eric’s body and sacrificing himself.
As Mia watches the cabin burn, blood rains from the sky. Since four souls have been claimed, the Taker of Souls rises as the Abomination which takes the form of how Mia was when possessed, going after Mia to claim her soul to complete itself. Mia attempts to flee in the car, but fails. She instead crawls into the shed through a hole and grabs a chainsaw to fight back. When the Abomination throws the car at Mia, she gets her hand crushed underneath. She tears off her hand and slices the Abomination in half with the chainsaw. Its corpse sinks into the ground and the rain stops. Mia stumbles away into the woods. Unknown to her, the Naturom Demonto is still intact.
In a post-credits scene, a silhouetted Ash Williams says “Groovy” and looks at the camera before it cuts to black.