At the North Bend Psychiatric Hospital in 1966, a sinister event unfolds when a young patient named Tammy is horrifically attacked and killed by an unseen force during the night. The following morning, we meet Kristen, portrayed by Amber Heard, a troubled yet beautiful young woman who is found bruised and cut while setting fire to an abandoned farmhouse. The local police intervene and bring her to the same psychiatric hospital where she soon starts to confront the dark and mysterious happenings surrounding her.
Upon her arrival, Kristen is assigned to Tammy's former room and meets the other patients, including Iris (Lyndsy Fonseca), Sarah (Danielle Panabaker), Emily (Mamie Gummer), and Zoey (Laura Leigh). After receiving medication to help her rest, she experiences strange occurrences at night — most notably, her blanket being pulled under the bed. In her quest to reclaim her comfort, Kristen discovers a broken charm bracelet hidden beneath her bed, which leads her to deeper suspicions.
The next day, she comes face-to-face with Dr. Stringer, played by Jared Harris, who points out that she had the address of the burned farmhouse written on her hand and questions her about the events. Kristen struggles to recall the fire, adamantly insisting she is not insane. As night falls, she attempts to escape but is caught and returned to her room, only to awaken later and see a horribly disfigured figure watching her through the window.
In the courtyard with Iris and Emily, Kristen spots two mysterious individuals in Dr. Stringer's office. When she inquires about them, Iris and Emily seem evasive, hinting that they have seen these people many times before. During a shower, Kristen is suddenly attacked by the disfigured figure. After reporting her attack, she is met with skepticism — the staff drugs her and subjects her to rigorous electroshock therapy.
The plot thickens as Dr. Stringer conducts a therapy session where the existence of Tammy is brushed aside. Iris's last therapy session quickly escalates when she undergoes hypnotherapy, only to later disappear after being kidnapped and killed by the very same disfigured figure. Concerned for her friend, Kristen searches for answers, discovering Iris's sketchbook filled with eerie drawings that include a girl named Alice, alongside the ghostly figure that has haunted her.
Haunted by visions of Alice Hudson, Kristen confronts Dr. Stringer with her theories, but he dismisses her claims about the haunting. Emily reveals some shocking truths — that Alice once lived among the girls in the hospital but escaped from its confines. As the tension builds, Kristen and Emily attempt to flee the ward, but their plans are thwarted, leading to Emily being captured after a failed escape attempt.
The chilling tale continues as Kristen learns of Alice's tragic fate; understanding that all the girls were complicit in Alice's suffering. In a horrifying turn of events, Emily, overwhelmed, takes her own life with Alice's ghost guiding her. Desperate to escape the malevolent presence, Kristen holds Zoey hostage, but her plan is once again foiled by the resilient Dr. Stringer and his orderlies. As Kristen finds herself imprisoned in a straitjacket, she manages a clever escape alongside Zoey, yet danger stalks them relentlessly through the corridors of the hospital.
In a frantic pursuit, Zoey is captured, and Kristen finally confronts the ghost, managing to kill it. However, the shocking twists do not end there. In the aftermath, Kristen uncovers a file belonging to Alice Hudson, which reveals dark secrets about her own past.
Ultimately, the truth unfolds like a dark tapestry; Kristen's true identity is revealed as Alice Hudson herself. Flashbacks showcase the horrifying past of a young girl who was kidnapped and held captive in a basement for an extended period. Suffering from multiple personality disorder, Alice had developed personas representing her trauma, leading to the creation of Kristen and other alter egos for self-preservation. Dr. Stringer attempts to navigate this fragile psyche, revealing that Kristen is merely a protective manifestation of Alice's trauma.
In a final tumultuous confrontation, Alice's ghost reemerges, leading to a chaotic climax that results in her and “Kristen” falling from a window, culminating in Alice’s reawakening. In the end, Alice’s parents return to take her home, but the horror is not fully over as Kristen re-emerges, ready to attack once more, leaving the audience questioning the boundaries of identity and the lasting scars of trauma.
