After the suicide of her husband Jonathan Grey, college student Sarah Foster begins to experience dangerous episodes of sleepwalking. Her former professor, Dr. Cooper, suggests a stay at a sleep clinic, where Dr. Koslov and Dr. Scott White monitor her brain activity. Following a night of dreamless sleep, she wakes in a different room, and Koslov informs her that she has been moved for further observation and should stay longer. The unsettling switch between realities becomes the first hint that her perception is not stable.
Soon, she encounters a troubling social mismatch: a professor and her roommate, Dawn, call her “Miss Wells” instead of Foster, and her personal documents identify her as “Sarah Wells.” The mystery deepens as Koslov presents a form about a dream of being chased, though she insists she has not had any dream and that the handwriting on the form resembles hers, even if she cannot accept it as hers. The sense of losing control grows as her nights of dreamless sleep give way to fragments that blur memory and identity.
In spite of her efforts to keep a grip on reality, Sarah learns about selective memory loss and partial retrograde amnesia, a clue that life-altering gaps can exist in the mind. Back at home, her apartment reveals another shock: her roommate has been replaced by a stranger named Nichole, and a phone call from a man who claims to know her adds to the confusion. Dr. Cooper does not recognize her, leaving her more alone in the maze of memory and dream.
That night, she dreams of suffocation, a haunting omen that echoes through the days that follow. The next morning, she wakes up in a strange house, locked out of her own apartment, and finds Nichole and another girl inside. After breaking in through a window, she discovers unfamiliar belongings and a menacing taunt over the phone. She hides in the laundry room as water seeps under the door, a surreal invasion of her private space that mirrors her fractured mind.
When she returns to the clinic routine, everything seems normal again and Dawn is back on the scene. Sarah seeks help from White, who reaches out to Cooper, and Cooper begins to remember her. White explains that the sleep issues can warp perception and that dreams might serve as a bridge to understanding emotions. Outside, a dark-haired man begins to chase her, but White arrives to offer a ride and to illuminate how dreams can reveal buried feelings and motives.
Her dreams intensify: she envisions being trapped in plastic and wakes again in the strange house, this time confronted by a fearful woman who feels all too real. Back in her own apartment, the strange belongings reappear, and in the lab Koslov does not recognize her, while another man named White claims to know her. During a campus lecture, a woman in the audience recognizes Sarah, further destabilizing her sense of self. She evades the police outside her apartment and returns to the laundry room as water again begins to seep in around her.
Eventually, Sarah wakes at the sleep laboratory to hear White confirm that the police found her sleepwalking. Though she cannot recall the events, the data clearly shows she was dreaming. White suggests that the stalker might be the key to linking the two competing realities. Reading through her diary, she discovers a shocking truth: Jonathan was murdered, even though she believed he had killed himself.
Back in her apartment, an assault by a dark-haired man disrupts the fragile sense of normalcy, but White arrives to ensure she remains unharmed. Cooper advises Sarah to seek protection while her condition stabilizes, and the pieces begin to align: the stalker is named Warren Lambert, and memory surfaces in the wake of another attack. The danger becomes personal as the case unfolds, and Sarah finds herself drawn to an intimate moment with White, even as the lines between dream and waking life blur further.
During sleepwalking, Sarah encounters the woman from her dreams, realizing she is part of that other woman’s nightmare rather than a separate foe. The truth comes into sharper focus: she is actually Anna Wells, the obsessive fan who shot Jonathan. The original Sarah—the real woman—was the one Jonathan ended up with. Flashbacks reveal Anna in a coma, gasping for breath while contending with an identity she cannot bear, trapped inside a mind that mistakes her own past for another life altogether.
