Julia Streak [Louisa Krause] is a troubled, antipsychotic-dependent young woman who takes a job as a night guard at a grand but abandoned apartment complex so she can support her daughter, Clara [Morgan Waulters]. She is accepted by master keeper Dixon Boothe [Ezra Knight] and introduced to the only other security guard for the complex, the rude paraplegic Dennis Cooper [Jason Patric], who has been working at the complex since it opened. While patrolling, Julia hears whispers from Room 441, whose access is blocked by a locked door. Cooper denies that there is anything unusual behind the door, claiming that the owners left the room unfinished.
Sometime later, a homeless man, Jim [Mark Margolis] attempts to enter the complex. Cooper refuses him access, but Julia allows him shelter under the condition that he does not wander. Using a hammer from Jim's belongings, Julia breaks the lock and discovers that camera 441 is a corridor leading to a dormitory. Meanwhile, Jim wanders anyway and follows Julia into the dormitory. There, Julia encounters a number of deformed children. She calls for Cooper's aid, but the two end up losing track of Jim. Jim, now at the dormitory, is then killed by a blonde girl, who smashes his head with a faucet.
Julia searches through the internet and finds a video of the Wellville, a center for deformed and mentally-challenged children, which had been accused of abusing its patients. Julia connects the center with the dormitory but Cooper, having found out that she is dependent on anti-psychotics, handcuffs her while he agrees to go down to Room 441 alone. However, his wheelchair is forced to the dormitory, where he is haunted by the deformed children.
Following Cooper, Julia meets an apple-cheeked boy [Henry Kelemen], who gives her more details about Wellville: the children, four in total, attempted to escape, but they were caught and forced inside a room containing a reservoir of contaminated water. By the time that the Wellville's crimes were revealed, all but one had died. Thinking that the survivor struggling to get out of the reservoir is Clara [Morgan Waulters], Julia jumps inside, despite Cooper's warning. She is saved by Cooper from drowning at the cost of his life. Finding the door locked, Julia asks for mercy and is confronted by the children, including the face-deformed blonde girl. She convinces the girl to forgive her, after which the children allow her to leave.
It is revealed that all the events were happening inside Julia's mind. Julia is in fact the deformed girl, being the daughter of "Dennis Cooper", whom she gave a new identity in her dream. Dixon is the doctor assigned to her, Jim a sickly old man being treated next to her, while Clara is the name of her doll. The film ends with Julia flatlining after being on life support since she was a child, next to her grieving father.
