Mia, a young woman struggling with severe drug addiction, travels to a dilapidated family cabin in the woods alongside her estranged brother David, David's girlfriend Natalie, and their childhood friends Olivia and Eric. The group plans to stage an intervention and keep Mia locked in the cabin while she detoxes cold turkey.

When a foul smell leads them into the basement, they discover decaying animal carcasses, an altar, and a book wrapped in barbed wire: the Naturom Demonto (Book of the Dead). Despite explicit handwritten warnings scribbled across the pages, Eric deciphers and whispers an incantation aloud, awakening a demonic presence in the forest.

Mia begins experiencing horrific visions and smells rotting decay, but the group brushes off her panic as withdrawal symptoms. Terrified, Mia steals David's car to flee, crashes into a flooded creek, and is ambushed by the malevolent entity through tangled vines. Possessed, Mia returns to the cabin, scalds herself with boiling water, shoots David with a shotgun, and warns them that they will all die tonight before collapsing.

David and Eric lock the possessed Mia in the storm cellar. Soon, the demon spreads to the others. Olivia becomes infected, mutilates her own face with broken glass, and attacks Eric before he beats her to death with a toilet tank lid. Natalie gets bitten on the arm and amputates her own limb with an electric carving knife to stop the rot, but the infection takes hold anyway, forcing David and Eric to put her down.

Eric reveals that according to the text, the demon requires five souls to unleash an abomination called the Taker of Souls. After Eric dies from his injuries, David sedates Mia, buries her alive to purge the demonic spirit as prescribed by the book, and digs her up, successfully reviving her with an improvised defibrillator. Just as the siblings prepare to leave, a dying, possessed Eric sets the cabin ablaze. David sacrifices himself inside the burning structure to ensure Mia escapes.

Outside, the fifth soul has been claimed by the forest, causing the sky to rain blood as the monstrous Taker of Souls crawls up from the earth. Arming herself with a chainsaw, Mia fights the demon in the crimson downpour. Even after her hand is pinned and severed under David's overturned truck, Mia tears herself free and drives the running chainsaw straight through the abomination's mouth, splitting it in two. The demon collapses into the earth, the storm clears, and an exhausted, mutilated Mia walks alone down the road into the dawn.

The climax centers on the Book of the Dead’s ritual prophecy: five souls must be consumed by the demon to resurrect the Taker of Souls from the underworld. David fulfills the ritual conditions without realizing it by dying inside the burning cabin. When the fifth soul is claimed, a literal blood rain begins, signaling the arrival of the red-robed demon who resembles Mia's demonic reflection.

Mia survives by enduring severe self-mutilation: she yanks her pinned hand completely off at the wrist to reach the chainsaw. By slicing the demon vertically from mouth to groin, she purges the evil back into the earth. The blood rain stops, leaving Mia alive as the sole survivor. In a post-credits stinger, original franchise hero Ash Williams (Bruce Campbell) turns toward the camera in profile and utters his signature catchphrase: "Groovy."