A young woman becomes a prisoner in the cellar of a secluded cabin nestled deep within the woods. Here, an old woman recites incantations from an ancient book written in Welsh. As the young woman passionately pleads for her mother's help, she reveals that she is possessed by a sinister demon. In a desperate attempt to save her, her father, Harold, horrifically sets her ablaze before he is forced to shoot her down.
Some time later, David, accompanied by his girlfriend Natalie and their dog Grandpa, arrives at this very cabin, which is known to be his family's holiday retreat. They are joined by David's estranged younger half-sister, Mia, and their friends: high school teacher Eric and nurse Olivia. The group's purpose for this trip is to assist Mia in overcoming her heroin addiction. Olivia cautions David, informing him that this is not Mia's first attempt at rehabilitation, while Eric reveals that during Mia's previous effort, she experienced a near-fatal overdose before being revived.
Upon exploring the cabin, David stumbles upon evidence of a recent break-in. In the filthy cellar, he discovers an unsettling collection of rotting animal corpses, a worn double-barrel shotgun, and a mysterious book known as the Naturom Demonto. When Eric reads an incantation from this book, they unwittingly awaken a malicious force. Mia soon finds herself tormented by a demonic doppelgänger in the woods. In her desperate attempt to escape, she crashes Eric's car into a nearby swamp and is subsequently ensnared by vines that assault her, leading to her possession.
As events unfold, David finds Grandpa brutally beaten to death and catches Mia in a shocking act of self-mutilation, scalding herself in the shower with boiling water. Although David attempts to take Mia to a hospital, they are thwarted by flooded roads, and Olivia, concerned for Mia's well-being, sedates her. That night, Mia inflicts a wound upon David with the shotgun and performs a grisly act of vomit transfer with Olivia before Eric confines her to the basement. The situation escalates as the possessed Olivia violently mutilates her own face and ultimately stabs Eric, who is forced to kill her in self-defense.
Mia's sinister influence grows as she lures Natalie into the cellar, biting her hand and inflicting a horrific injury upon herself. David manages to rescue Natalie, but Eric explains that per the lore in the book, the Taker of Souls must claim five victims to unleash the horrifying Abomination. To save Mia, the group realizes they must either bury her alive, dismember her, or incinerate her. Meanwhile, Natalie's wound becomes increasingly infected, leading her to attempt an amputation with an electric knife, which causes a power outage in the cabin. Now fully possessed, she attacks David and Eric with a nail gun, resulting in her gruesome death after David shoots off her other arm.
Faced with despair, David resorts to dousing the cabin in gasoline, but ultimately he decides to bury Mia alive instead. After digging a grave, he faces a grave situation as Mia attempts to drown him, leading to Eric's fatal sacrifice to save him. David manages to sedate and bury Mia, yet upon realizing her heartbeat has ceased, he digs her back up and tries to revive her with a makeshift defibrillator, successfully bringing her back to life. The enraged Taker of Souls, however, possesses Eric's lifeless body and confronts David in the cabin as he retrieves his car keys. In a final act of desperation, David shoots a gasoline can, sacrificing himself to vanquish the possessed Eric.
With David's sacrifice claiming the fifth soul, blood starts to rain from the sky, heralding the rise of the Abomination. In a climactic showdown, Mia uses a chainsaw, obtained from a tool shed, to sever the Abomination's legs. However, the creature retaliates by crushing her left arm under the weight of David's Jeep. Despite this setback, Mia manages to free herself and ultimately bisects the Abomination's head and torso with the chainsaw, causing it to sink back into the earth and halting the blood rain. Exhausted but triumphant, Mia escapes the chaos, blissfully unaware that the Naturom Demonto still remains intact, carrying the potential for further horror.
In a post-credits scene, we see an older version of Ash Williams, the iconic hero from the original Evil Dead trilogy. In a darkened silhouette, he utters his renowned catchphrase: "Groovy."
