A group of friends arrive at a cabin in the woods, planning on a few nights of relaxation, conversation and to help the youngest member, Mia overcome her addiction to heroin. Upon arrival, they discover the cabin is littered with corpses of dead animals as well as a shotgun and a book entitled 'Naturom Demonto'.
Eric decides to read the incantation, despite all warnings and awakens a menacing force. Mia begins seeing visions of a bloody girl in the woods. After expressing her desire to leave, the group come to the conclusion that Mia is experiencing hallucinations due to heroin withdrawals. Mia steals the group's truck and drives through the woods, before crashing after seeing the demon girl appear in front of the car. She begins to run through the woods, being chased by the demonic being. She falls and becomes tangled in possessed vines. She realises the demon girl is a version of herself. A vine appears from the demonic version of Mia and proceeds to rape Mia, infecting her with the possession. Mia's brother, David and Olivia from the group find Mia and take her back to the cabin.
Later that evening, David discovers his dog has been viciously killed with a hammer and he goes to confront Mia about it. In the bathroom, the water has been turned on so hot that Mia is scalding herself. David tries to drive her to hospital but the river has flooded the roads, making it impossible to leave. Meanwhile, Eric becomes increasingly sure the book he read from is the cause of Mia's strange behaviour and the group's ensuing bad luck.
During the course of the evening, Mia is clearly possessed. She grabs the shotgun and shoots David in the arm, warning the group that they will all die before passing out. Olivia tries to get the gun away from Mia who suddenly wakes up and overpowers Olivia. Before Olivia can get the gun, Mia projectile vomits blood over her. Eventually, Olivia shoves Mia off and tumbling down a flight of stairs to the basement where Eric locks her in. Olivia leaves to clean herself up in the bathroom but is left terrified at her own disfigured reflection in the mirror. Before she can warn the others, she freezes by the door and becomes possessed.
Eric finds her in the bathroom, slicing at her face with the broken mirror. Olivia attacks him and stabs him in the chest. After a brief fight, Eric manages to bludgeon Olivia to death with a broken piece of sink. Eric is taken to the shed where David tends to his wounds. Eric is convinced he released something evil after reading the book.
Meanwhile in the house, Natalie is lured into the basement by Mia where she bites into her hand. Natalie tries to attack her with a box cutter but possessed Mia snatches it off her, cutting her own tongue in half. She plants a bloody kiss on Natalie's mouth before David interrupts, allowing Natalie's escape. The demon tells him that Mia no longer exists. David chains her up and locks the basement.
Eric explains that the book states there must be five souls claimed by the Taker of Souls to release the Abomination. Natalie is in the kitchen, cleaning her wound and becomes convinced her arm is infected. She decides to cut it off with an electric knife. David tries to patch up the wound whilst Eric explains that Mia needs to be cleansed, either via fire, live burial or dismemberment. Natalie, suddenly possessed, attacks them with a nail gun but is stopped when David shoots her remaining arm off. She returns to normal and collapses dead in David's arms.
Eric has convinced David that they need to kill Mia in order to stop the events. They burn Olivia's corpse and dismember Natalie's whilst planning to burn down the cabin with Mia locked in the basement. They stop as Mia begins singing a song from childhood, causing David to change his mind and bury her instead. He goes down to the basement to try and subdue her, but she attacks and tries to drown him. Eric manages to stop her but is stabbed again and succumbs to his injuries. David sedates Mia and begins to bury her in a grave outside. He unearths her and uses the defibrillator from the cabin to bring her back, successfully exorcising the demon and healing Mia. The siblings reconcile and decide to leave. As David goes back into the cabin to get keys, the possessed body of Eric attacks him with wire cutters and stabs him in the neck. Mia tries to help but is urged to leave the cabin.
Mia watches the cabin burning down before it mysteriously starts raining blood. Five souls have been claimed, meaning the Abomination can come back for Mia. She attempts to flee in the car but the demon smashes through the window. Mia escapes into the shed, finding a chainsaw. She uses it to attack the Abomination. It throws the car at her, which she manages to dodge but gets her hand stuck underneath it. Knowing she'll die if she doesn't escape, Mia tears her hand off and grabs the saw. Mia then saws the Abomination in half. The blood rain stops and Mia wanders off into the distance.
In a post-credits scene, an older Ash Williams is seen in a shadowed profile. He mutters out his catchphrase "Groovy..." before he turns his head to the camera.
