|“||Feast on this, motherfucker!.||”|
— Mia (to Abomination Mia)
Mia Allen is the younger sister of David and childhood friend of Olivia and Eric; though they only knew one another for a short time, Mia presumably had a good opinion about David's girlfriend, Natalie. Early on, Mia is revealed to be a recovering drug addict and the reason the group travel to the cabin in the first place, in order to help Mia quit her addiction.
In the first half of the film, a possessed Mia serves as the main antagonist, only to be killed, then revived by her brother, assuming her role as the heroine of the film. Mia is the only survivor of her group of friends, who were all subsequently dispatched by demonic forces originating from the Naturom Demonto.
In the screenplay, Mia is revealed to be twenty-six years old, having been born in 1987 to her unnamed parents. Dialogue between the group reveals that David had seemingly abandoned Mia, leaving her to care for the ailing mother, who mistook her for her brother just before she passed. This caused a rift in their relationship and possibly fueled Mia's drug addiction. Mia is revealed to be superstitious and believes in protection from supernatural forces, in contrast with her brother, who is more skeptical of such things. Early in the film, Mia is tormented by an unseen presence, which appears to her as a demonic doppelganger of herself, possibly a physical representation of her addiction. Mia begs to leave the cabin, but the others refuse, believing she is hallucinating due to withdrawal.
A group of young adults: David Allen, his girlfriend, Natalie, childhood friends, Olivia and Eric, and David's younger sister, Mia, travel to an isolated cabin in the woods, once used by David and Mia's parents as a vacation spot, in order to help Mia quit her drug addiction. Upon arrival, Mia dumps the last remnants of her heroin down an old well; the group soon discovers rotting feline corpses, a shotgun, and a book, entitled the Naturom Demonto, wrapped in plastic and barbed wire. Eric reads a passage from the book, conjuring up a malevolent force, which begins tormenting Mia, appearing as a demonic doppelganger of herself. Mia begs for them to leave the cabin, but the others refuse, largely due to the fact that they believe she is only suffering from withdrawals. An agitated Mia manages to escape the cabin by climbing out a window and leaving in Eric's vehicle, but swerves upon seeing a supernatural figure in the road, crashing into a nearby marsh. When she attempts to swim to safety an unseen presence emerges from the water and pursues her through the woods; Mia becomes disoriented and tumbles into a brier patch. As she attempts to crawl to safety, the briers seemingly tighten around her body, preventing her from escaping. Her doppelganger appears once again, vomiting up a slimy appendage, which rapes Mia, effectively possessing her.
Moments later, Mia is discovered by her friends in a disoriented state. They bring her back to the cabin, where she relays her encounter with the demonic forces in the woods to David, who dismisses this as another hallucination brought on by her drug withdrawal. David later discovers his dog, Grandpa, has been killed, bludgeoned with a hammer. Realizing Mia is responsible, he goes to confront her, but discovers her scalding herself in the shower. David attempts to drive her to a nearby hospital, but discovers the roads are inaccesible to due to the rain. He brings her back to the cabin, where Olivia uses her knowledge as a nurse to care for Mia's burns. As the group discuss what to do about their predicament, Mia enters the room and shoots David in the shoulder; suddenly, an unseen force enters the cabin, as a demonic voice warns the party that they will all soon be dead. Mia passes out and Olivia goes to retrieve the gun used to shoot David, only to be ambushed by a possessed Mia, who pins her to the floor, before vomiting blood all over her.
Olivia is able to push Mia down into the cellar, before David locks her in. Olivia goes to wash the blood off herself, only to becomes possessed herself. When Eric goes to check up on her he discovers she has mutilated her face with a piece of glass; the possessed Olivia attacks Eric, who kills her by bludgeoning her with a broken piece of the sink. David treats Eric's wound and instructs Natalie to retrieve some supplies from the kitchen, only for her to be lured into the cellar by Mia, who bites her on the hand, before slicing her tongue in half with a box-cutter and forcefully kissing her. David manages to rescue Natalie, before chaining down the hatch to the cellar.
Some time later, Natalie's hand begins to decompose and ooze black fluid, indicating her impending possession; she manages to amputate her arm using an electric carving knife, causing the cabin to lose power in the process. While this is happening, Eric reveals to David that once five souls have been claimed, an entity known as the Abomination will be able to take a physical form, rising from the depths of Hell. He also reveals the only way to save someone who has become possessed is by immolation, dismemberment, or live burial. Suddenly, a possessed Natalie enters the room and attacks them with a nail gun, before bludgeoning Eric with a crowbar, splitting his hand open. David subsquently dispatches her with the shotgun and Natalie dies in his arms.
David douses the inside of the cabin with gasoline, planning to escape with Eric in tow, but reneges on the idea to save Mia. He enters the cellar to subdue her, but is instead attacked and nearly drowned by her; however, Eric is able to incapacitate her, but is stabbed in the process and succumbs to his wounds. David buries Mia alive, before digging her back up and reviving her using a makeshift defibrillator. A revived Mia waits for David while he enters the cabin to retrieve the keys to his Jeep, only for a possessed Eric to appear and wound him with a pair of pliers. A dying David locks Mia out of the cabin, before shooting a canister of gasoline, immolating himself and Eric.
Blood begins to rain down from the sky, as Mia retrieves the necklace given to her by David at the beginning of the film. Suddenly, the Abomination rises up from the ground, appearing as Mia's demonic doppelganger. She attempts to escape in David's Jeep, but is ambushed by the demon. Taking refuge in the shed, Mia arms herself with a chainsaw, which she uses to sever the demon's legs. Following a prolonged fight, Mia's arm is pinned beneath David's Jeep, forcing her to sever her own arm to escape. As the Abomination crawls towards her, Mia shoves the chainsaw through the demon's head, bisecting it. The Abomination, now vanquished, becomes submerged in a pool of blood, disappearing into the ground. A traumatized Mia limps away from the cabin to search for help; meanwhile, the Naturom Demonto is shown lying open nearby, before flipping and closing on its own.
In the screenplay, Mia is revealed to have made it out of the woods to a nearby road, where she passes out from her wounds; an unnamed motorist stops to assist her, taking her to a hospital.
- Similarly to Ash, Mia uses a chainsaw to defeat the demons of the Naturom Demonto.
- Mia is a composite of Ash Williams and his sister, Cheryl. Mia is more similar to Cheryl than Ash, despite being the heroine of the film; she is the main protagonist's sister and the first of the five characters to become possessed, taking on the role as an antagonist for the majority of the film. However, Cheryl is never revived and is killed during the climax of the film. She is similar to Ash in that she uses a chainsaw to defeat the Abomination and also says a one-liner: "Feast on this, motherfucker!"
- Mia's one-liner: "Feast on this, motherfucker!" is similar to Ash's: "Swallow this." Both say these before defeating their respective foes.
- In the video game, Resident Evil 7: Biohazard, a character named Mia attempts to kill her husband, Ethan, with a chainsaw, after becoming possessed by Eveline.