The synopsis contains information that may give away important plot points. You may not want to read any further if you've not already seen this title.
The film starts with a young woman walking through the woods. She's dripping with blood. A figure stalks her and eventually jumps her, throwing a bag over her head and having his redneck buddy whack her with the butt of his axe. The woman wakes up in a cellar in the presence of a woman who speaks only Welsh, and she's surrounded by what appear to be burn victims. The young woman's father appears before her. She asks for her mother, and the father tells her that she killed her mother. The old woman hurries the father to kill his daughter, and so he douses her with gas as she pleads with him. Before he can strike a match, the young woman tells him she will eat his soul. Her eyes turn yellow, and as the father drops the match, she appears demonic and starts thrashing. The father raises a shotgun, tells her he loves her, and then blasts her head clean off her shoulders.
Some time later, a car is driving to a quiet part of the woods where a cabin is. We meet David (Shiloh Fernandez), his girlfriend Natalie (Elizabeth Blackmore), and their friends Olivia (Jessica Lucas), who is a nurse, and Eric (Lou Taylor Pucci). Olivia tells David his sister Mia (Jane Levy) is waiting for him in the back.
David finds Mia, sitting on an old broken down 1973 Olds Delta 88, smoking a cigarette while drawing. He tells her she looks beautiful, but she thinks he's being a liar.
The group enters the cabin, where Mia complains about a smell. We learn here that Mia fondly remembers her mother, but David doesn't want her to. It also appears that their mother was emotionally abusive toward Mia.
Everyone gathers around a well. Mia, being a recovering drug addict, pulls out a bag of powdered drugs and pours it down the well, vowing to be done with it for good and going cold turkey.
Olivia tells David that this isn't the first time they've tried to get Mia off drugs. Eric tells him that she lasted eight hours before relapsing and overdosing, in which she legally died but was revived.
That evening, Mia starts screaming as she is apparently suffering from withdrawals. Olivia gives her a sedative but Mia continues complaining about a rotten smell that nobody else seems to notice. Their dog, Grandpa, sniffs around the floor where they find a door leading to a cellar under a rug. The floor is smeared with blood. David and Eric go into the cellar and discover the stench is coming from the rotting corpses of animals. They also find the "evil book" from before, wrapped in wires and plastic.
The next day, Mia is wandering outside in the rain. Eric opens the cover of the book, which is scribbled all over with warnings like "Leave this book alone" and "Do not fucking read this." Naturally, Eric looks through it and finds a page with some words scribbled out and the warning to not speak, write, or hear what is in it. He puts a piece of paper over the book and shades over it with a pencil, revealing several words that he begins to read. Outside, Mia starts being sick andd hears a faint voice calling her, and then sees a figure staring at her. She comes into the cabin and starts grabbing her things, saying she can't be there anymore. She jumps out the window and steals Eric's car, but as she drives through the woods, she sees the figure again and drives the car into a swamp.
When she comes to, Mia tries to get out, but the figure rises from the swamp, and she runs away, tumbling into a thorn bush. The branches and sticks wrap themselves around her arms and legs, and the figure, which is a bloodied and horrifying version of herself, regurgitates a black wormy thing with thorns that makes its way up Mia's leg and goes right inside her. David and Olivia hear her screaming and find her.
The group suspects she was trying to harm herself. David goes into her room, and she tells him that there is something in the room with them.
He goes outside and sees blood on the ground. He calls for Grandpa and finds him in a hole beneath the tool shed. Grandpa is whimpering. David runs into the tool shed and pulls him out of the hole, but he's dead. David suspects Mia bludgeoned him to death with a hammer that was lying nearby.
He runs back into the cabin and tries to get to Mia, but she's taking a shower. She turns the water up to the point where it becomes scalding hot and her flesh starts burning. The group pulls her out in time. Eric looks at an open page in the book and notices a picture of a being with burning flesh.
David takes Mia in his Jeep and drives her to a hospital as she's foaming at the mouth. Unfortunately, the roads are flooded, and he has to turn back.
Olivia gives Mia another sedative. While the group argues about her, she comes into the living room dragging a rifle. She holds it up, and fires near David, and the door swings open as Mia lets out a frightening scream. She says in a possessed voice "You are all going to die tonight." Olivia tries to get the rifle away from her, but Mia tackles her as her eyes turn yellow and she spits blood all over her. Olivia kicks her into the cellar, and Eric closes it. He suspects this has to do with the stuff they found down in the cellar.
Olivia tries to clean the blood and throw up off herself, but she sees an image of herself mutilated in the mirror, which shatters. She walks away but then freezes, and her eyes twitch as she wets herself. Next to her is the book, open to a page with a person holding a knife and their severed face flesh.
Eric goes into the bathroom to find Olivia, who is cutting the flesh from her cheek with the broken glass. Horrified, he stumbles backward and slips on the cheek flesh. Olivia stabs him with the broken glass and then in the face with the needle. Eric throws her off and breaks off a piece of the toilet, which he uses to bludgeon her to death, just as David and Natalie come in.
David tries to patch up Eric's wounds. He tells Natalie to bring a jug of water and sugar. Eric tells her to stay away from Mia. He tells David that it's his fault this is happening because he read from the book.
Natalie goes to the living room and finds that the cellar door is open. Mia is still down there, crying and asking for help. As Natalie tries to go down and get her, Mia's voice becomes low and demonic and threatening, saying "He won't stop until he has all of you!" Natalie tries to run but Mia grabs her and bites into her hand. She tries to defend herself with a box cutter but Mia takes it from her and slices her tongue down the middle. She grabs Natalie and kisses her, forcing blood into her mouth. David finds them and pulls Natalie out. They nail the cellar door shut, locking Mia down there.
David finds Eric trying to burn the book, but it doesn't work. He tells David about what he knows from the book - an evil entity has been unleashed, and it's attached to Mia's soul. It seeks a collection of souls to unleash something called the Abomination.
Meanwhile, Natalie tends to the bite wound on her hand. It starts to burn into her arm and she notices an electric knife and reaches for it. Mia peeks from the cellar and tells her not to cut it off, but Natalie slices into her arm. David and Eric find her as her arm falls off.
Eric tells David there are three ways to put an end to this - bury Mia alive, dismember her, or burn her alive. David refuses to do any of that, and Eric calls him a coward. Suddenly, a noise is heard, and Natalie comes in, now possessed, with nails in her head. She's holding a nail gun and shoots at Eric, who gets a few nails in him. David tries to get the rifle, but Natalie starts hitting him with a crowbar. Eric shoots her with the nail gun, and she goes to start bludgeoning him with the crowbar. Before she can deliver the fatal blow, David blows her hand off with the rifle. Natalie appears to revert back to normal, complaining about the pain before she dies.
David finally decides to burn the cabin with Mia still inside. He pours gasoline on the floor, but before he can drop the lighter, Mia, in her normal voice, starts singing a lullaby that their mother used to sing, and David cannot do it. Outside, a bolt of lightning strikes a tree, setting it on fire. He hatches another plan. He grabs two syringes, among other things, and starts to dig up a hole outside.
He heads into the cellar. He finds Mia, who slashes at him with the box cutter and throws him around. She tries to drown him, but Eric steps in and hits Mia. He's also been stuck with the box cutter in his stomach. David goes to him, and Eric finally dies.
David takes Mia outside with a bag over her head and starts to bury her. She talks to him in her normal voice and pleads with him, but he won't buy it. She starts telling him about their mother and more of her abusiveness. Their mother kept asking for David, who was never there, and how Mia would have to keep telling her he's coming back. He completely buries her, and then waits a while as the rain stops and the fire on the tree goes out. David digs Mia back up and pulls her out. He grabs a car battery with the syringes attached to it and sticks them in Mia's chest, trying to revive her. She doesn't come back. He covers her body and walks away tearfully. However, Mia rises and talks to David. She is completely back to normal, and they hug.
The two go back into the cabin to get the keys to David's Jeep, but a possessed Eric is behind him and he stabs David in the neck. David goes into the hallway and gets Mia outside and locks her out. He grabs the rifle and shoots at the gasoline container, letting it consume the cabin in flames and killing himself and Eric for good.
Outside, it starts raining blood. A hand bursts from the ground and grabs at Mia. It's the Abomination. It chases after Mia, who grabs a chainsaw from the tool shed and tries to run. She hides under the Jeep and manages to slice off the Abomination's legs, crippling it. As she tries to run, though, the Abomination tips the Jeep over and it falls on Mia's arm. She pulls herself free, severing her arm. The Abomination inches toward Mia, but she sticks her arm into the handle of the chainsaw and kills the Abomination. Its corpse sinks back into the ground.
The bloody rain stops, and the sun comes out. Mia walks away, now free from the curse, but alone. Meanwhile, the book lies outside the burning cabin, closing itself and waiting for its next victims.
Ash (Bruce Campbell) appears after the credits to say "Groovy" and dramatically turn to the audience.