College student Sarah Foster is found by the police, as she is sleepwalking in her nightgown on the road. Since the suicide of her husband Jonathon, who worked as a novelist, she is suffering from sleep disorder. A few days later, she talks to Dr Cooper, whose student she was, about the sleepwalking and a recurring nightmare, in which she is attacked by an unknown man. Cooper sends her to a therapy in a sleep medicine sleep laboratory. During a walk on a cemetery, Sarah talks about it with her room mate Dawn, who shows a personal interest in her professor Owen. Then an attractive man gets out of a black car and Sarah imagines him being a single. At the evening in the sleep laboratory, Dr. Koslov explains to her that her neuronal activity will be observed during the night. He also introduces her to Dr. Scott White, the director of the lab. It is the man whom Sarah has seen at the cemetery. He tells her, that a student was buried and he was there with a colleague. Sarah confides to him that she loved her husband, but not his work as a novelist.The next morning she wakes up in a different room after a silent, dreamless night. White takes her case. He reports about irregularities in the theta waves and asks her to spend some more nights in the lab. Sarah recognizes that something is wrong.In the lecture hall she questions the statement of her teacher, who thinks that love stories are just a dopa mine kick or a bipolar disorder. But she is even more irritated when he addresses her as Miss Wells and a student repeats this name. Also Dawn, her driver's license, her diary and a dedication in her husband's book affirm this surname. Sarah is rejected by Cooper's assistant. In the sleep laboratory Dr Koslov shows her a protocol about her dream in which she is pursued. She denies having dream anything, but sees her signature on the form.After one more night under surveillance Sarah briefly mentions some kind of fear to White, but neither she nor the machines have recognizes a dream during the night. In the library she looks for books about psychology. She talks to an employee about forgetting things and he mentions partial retrograde amnesia as a cause for this; it may be cured by returning into everyday routine. But if there are also hallucinations, it might be a psychosis. In her apartment, Sarah does not meet Dawn, but a different young woman named Nicole, who does not know Dawn, but something about Sarahs sleepwalking. As Nicole leaves, Sarah gets a phone call from an unknown, scary voice, which introduces himself as her stalker in the dark. She goes to Cooper again, but the doctor does not remember her or the talk. At home she locks herself into her bedroom and says to herself that she is Sarah Foster. She has again a nightmare, in which she is attacked from behind and suffocated with a plastic bag. She wakes up in an unknown house, in whose letterbox she finds a letter addressed to the "current resident". In her nightgown she goes to her apartment, in which she sees Nicole and a second woman, but she cannot get in. She spends the rest of the night in a room next to the pool of the apartment complex.At daylight she sees the two women leaving the apartment. Then she smashes a window with a brick. In the apartment, she puts on a blouse and a skirt, which are too big for her size. The stalker calls her again on the phone and tells her, that he is going to watch her and wants to feel her. Sarah grabs some pills and alcohol and locks herself into a closet next to the washing machines. Water flows in under the door.After waking up, Sarah returns to her apartment, whose window is no more smashed. Dawn just comes out and in reaction to her question, Sarah states having spent the night at the sleep laboratory. She looks up Whites private address and drives there on her bike. On the way, she passes the unknown house, in which she woke up some time ago. White is renovating his house Ind welcomes her friendly. He listens to Sarahs report about the irritating events, and then calls Dr Cooper, who remembers Sarah. White tells Sarah, that she has unusual sleep patterns that might affect her perception. She asks whether the world might change dramatically, and runs away. Near her apartment she thinks that the stalker is behind her, but White is there again. He takes her to his home. She mentions that she has panic to fall asleep because everything could change again. White responds with the explanation, that chemical triggers in the brain cause emotions during dreams, which the rational part of the brain uses to construct a story. She insists, that the unknown house was real, but does not want to get in trouble with the resident. Watching a thunderstorm she asks herself, whether she can be rescued and why White helps her. He answers that they are going to stand this together and everything will make sense. Sarah spends the night in his house. In her nightmare she is covered up in plastic and then sees an unknown woman in bed, by whom she is rejected.In her nightgown Sarah leaves the mysterious house of her former dream and reenters White's house in the dawn, without meeting him there. She puts on yesterday's clothes, which are too big for her. In her own apartment she has a look at the window, which is now covered with cardboard. In the sleep laboratory, she asks for Dr White, but is not recognizes. A completely different man introduces himself as White. He knows a woman called Sarah Wells, but it is not the Sarah he is talking to. In the lecture hall, Sarah sees a quote which says that you do not need to know who you are, because the purpose of life is to become something different than at the beginning. In the group of students she also sees the unknown woman and panics even more. As cops are in front of her apartment, she hides in the closet and there is water on the floor again. Sarah wakes up in the sleep laboratory. The well known White tells her, that she has sleepwalked again, until the police brought her to his house. She cannot remember anything, but the data show that she has dream during the whole night. She talks about her last experiences in the other world. White thinks that the stalker might be an anchor connecting the two worlds. When he asks Sarah for the reason for her husband's suicide, she says, that she wrote about it in her diary, but cannot remember the event after her therapy. In front of her apartment, she reads her diary and is not irritated by Nicole arriving at the flat. On a public computer she looks at an old news article saying that the author Jonathan Gray was shot by a crazy fan. She shows the article to White. He says that he has all information about her husband's death from her and she herself learned everything about it from Dr Cooper, she says. After returning to her apartment she is attacked by the stalker, but she can free herself causing the attacker to go mad. When White arrives after being alarmed by her, the flat is undamaged and there is no trace of the attacker. Cooper joins the two and Sarah is confronted with another dream protocol. Although White tries to calm his patient, Sarah is put into a hospital and bound to the bed in spite of her resistance. At the same evening White meets Cooper who suddenly does not know Sarah and denies having seen White that afternoon. He drives back to the hospital to take Sarah, who is winding in her bed in spite of a strong sedation. White frees her and takes her to his house. There she again shows panic about falling asleep and he tells her about Coopers awkward reaction. Then he lies next to her to help her get into sleep.Sarah awakes with her head on his breast, still in the hospital clothes. In the following talk she states that she now remembers several things. Her stalker is called Warren Lambert and was obsessed with Jonathan, she says. The attack in her flat was not real; White explains it as a memory breaking through. Jonathon, she continues, had a sexual affair with a fan and Sarah witnessed it when she visited him in his office. She broke up with him, but the fan shot him, she says. In Whites dark house, Sarah is attacked by the stalker who tries to suffocate her with a plastic bag from behind. But White comes to help her and knocks the aggressor down. The policeman describes the attacker as a well known criminal and addresses Sarah as Foster. After that, Sarah and White have sex.In the night, Sarah sleepwalks again in her white nightgown. She sees the unknown woman first in the sleep laboratory and then in the house of the dream, where she asks for her name in vain. Through the rain she walks to the lab again. There the unknown woman wakes up seeing Sarah. The other White addresses the woman as "Sarah", but does not see Sarah in the nightgown. The normal White comes to Sarah and looks for an explanation. He finds a video statement of the unknown woman, who talks about feeling pursued and a woman standing next to her bed at night. In another flashback Sarah herself shoots Jonathon, who now talks with White's voice. In the lab Sarah says, that she is the other woman's nightmare. White's real version treats this woman, she thinks. But White insists that there must be an explanation. Sarah tells him, that Jonathon was her big love. But she is not his wife, but the other woman and she is called Anna. She shot herself. After this statement she talks to him about love and identity and he thinks they are in an in-between world. She nevertheless is sure that she created him to find out the truth.Then a woman wakes up in the ward for long-term coma of the Los Angeles university hospital. She is wearing an oxygen mask and is bound to the bed. A male nurse who looks like the stalker in the story comes to her. He addresses her as Anna and caresses her face. Water drops are falling on the woman's feet. Afterwards, the sleepwalker can be seen on the street again and she calls herself Sarah Foster.