The film opens with a young woman traipsing through the woods, her clothes stained with blood. As she moves cautiously, an ominous figure stealthily approaches her, ultimately catching her off guard with a sudden attack. He covers her head with a bag and, alongside his treacherous accomplice, strikes her with the butt of his axe, knocking her unconscious. Upon awakening, she finds herself in a dim cellar alongside another woman who communicates solely in Welsh, surrounded by the unsettling sight of burn victims.
Her father makes a sudden appearance, and in a chilling exchange, she inquires about her mother. Her father's chilling revelation reveals that she has killed her mother, inciting the old woman to demand her death. He reluctantly begins to pour gasoline over her while she desperately pleads for her life, ominously declaring she will consume his soul. Just as he prepares to ignite her, her eyes turn a terrifying yellow. The father's last words of love turn into horror as he pulls the trigger, ending her life in a shocking moment.
Fast forward some time, and we meet David (Shiloh Fernandez), his girlfriend Natalie (Elizabeth Blackmore), and their friends Olivia (Jessica Lucas) and Eric (Lou Taylor Pucci). They arrive at a secluded cabin in the woods, where David discovers his sister Mia (Jane Levy) waiting for him. As they gather in the cabin, Mia voices her discomfort about a lingering smell, and it's revealed that she is a recovering drug addict, vowing to rid herself of drugs for good by disposing of a bag of powdered substances into a nearby well.
Despite their concern for Mia, who has struggled with drugs before, the group seeks to support her. However, as night falls, Mia's detox manifests in terror; she screams, overwhelmed by withdrawal symptoms. An unsettling twist occurs when their dog, Grandpa, discovers a bloodied cellar, unleashing a chain of horrific events upon the group. They stumble upon an evil book, wrapped ominously, with warnings to leave it alone.
Despite these warnings, Eric cannot resist opening the book. As he reads from it, Mia begins experiencing unsettling visions that drive her to act recklessly. After she attempts to escape and drives her car into a swamp, Mia encounters the horrifying figure again and suffers an enraging transformation, as nature betrays her in the form of thorn bushes that bind her, leading to a demonic possession. The group suspects Mia is self-harming, but her quick spiral into madness leads to terrifying events, including a gruesome incident with their dog.
As the night escalates into chaos, Mia’s hold on reality weakens. After a shocking confrontation in the bathroom, where her friend Olivia mutilates herself, the remaining group members recognize the evil they have unleashed. With horrors unfolding at every turn, David realizes that Mia is possessed and they must act to contain the supernatural entity before it consumes all of them. As blood flows and fear escalates, the group's solidarity crumbles, leading to a desperate race against time.
In a shocking climax, David must confront the truth of what his sister has become. He fights to save her, but the evil entity—seeking to collect souls—drives him to the edge of despair. After the horrifying realization that they can only stop the entity by extreme means, David battles not just for Mia's life but to save himself and their friends from the Abomination's wrath.
> "You are all going to die tonight." Mia’s voice suffocates the air with dread, as the walls close in on them, shadows whispering of death. Amidst betrayals, battling the sinister evil, and ultimate sacrifice, David's journey culminates in an explosive final stand as he fights against time and fate to save Mia.
In a last-ditch attempt for survival and redemption, the cabin becomes a fiery tomb, while Mia musters her strength against the Abomination lurking to reclaim its prize. In a plot twist of horror and resilience, she fights her way free, wielding a chainsaw to sever the clutches of the entity.
When the storm subsides and sunshine breaks through the bloodied rain, Mia stands alone—liberated yet haunted by what transpired. As darkness lingers, the cursed book remains, patiently awaiting its next unsuspecting victims, leading to an iconic, wry nod from Ash ([Bruce Campbell]) as he proclaims, "Groovy."
