A man called Paul is working after hours and is murdered by a supernatural entity in the shadow. When his son, the boy Martin, is frightened by the same creature, he sees his mother Sophie talking to an imaginary friend called Diana in the shadow of her room. Martin does not sleep anymore during the night. His older step sister Rebecca who lives alone is summoned by the social assistant. She brings Martin home and recalls her own experience with Diana years ago when she was young. Rebecca and her boyfriend Bret investigate the connection of Sophie with Diana and come up to a scary revelation about their past.
The film opens with Paul (Billy Burke) skyping with his son, Martin (Gabriel Bateman), late at night while he's at work at a mannequin factory. Martin tells his father that his mother, Sophie (Maria Bello), has been acting strangely lately. Paul tells Martin that things will get better. Paul's assistant, Esther (Lotta Losten) is locking up the warehouse. When she goes to turn off the lights in one of the rooms, a figure appears in the back of the room. She notices the figure and when she goes to turn the lights back on, it disappears. She turns the lights back off and the figure reappears. She turns the lights on and it disappears. She does this two more times and on the fourth time she turns off the lights, the figure appears right in front off her. She immediately turns the lights back on and goes to tell Paul.She goes to tell Paul about the figure. She interrupts him as he's in the middle of his skype call with his son, and tells him that she saw someone who then disappeared. He dismisses it and tells her to go on home. A little while later, he goes to lock up for the night. As he is about to leave, he remembers what Esther told him about seeing someone. So, he walks through the barely lit warehouse. Eventually, he comes across the same figure. It's crouched down a darkened area of the warehouse. It notices Paul and starts to get up. Just then, one of the lights above Paul turns off, because it's motion-detected. Paul swings his arms around the light to get it to come back on. It does and now the figure is closer to him then it originally was. Paul sees this and runs away.As he passes through a small patch of darkness, the figure claws his leg, sending him to a lit part of the ground with a bloody gash. He gets up and sees the figure in front of him, standing in the darkness. He notices that it's standing right at the edge of the ground where the darkness ends. He pieces together that it cannot travel in the light, but only in the dark. Paul takes off again, limping all the way back to his office. He locks the door and grabs a baseball bat. Suddenly, the lights start flickering and eventually go off, leaving him in the dark. The handle on the door starts to jiggle as the figure tries to get in. Eventually, the door opens up slightly. Paul prepares for it to attack, but he's suddenly pulled from behind into the dark. His mangled corpse is dropped off somewhere on the other side of the warehouse.A few days later, Martin is about to go to sleep when he hears what sounds like his mother talking to someone. He makes his way through the hallway to her room, where it appears that she is talking to someone in the dark bathroom. When Sophie sees Martin standing by her room, she says "I'm sorry. Did we wake you?" He begins to get scared, but Sophie tells him to go to sleep. He begins to walk back to his room. He looks back to see Sophie standing at the door of her room and also sees a creepy hand at the corner of the door as well. Martin runs back to his room and locks the door. He gets back into bed. A few seconds later, the door handle to his bedroom begins to jiggle.On the other side of town, Martin's sister, Rebecca (Teresa Palmer), is hanging out with her boyfriend, Bret (Alexander DiPersia). The next day, Martin is in the nurse's office after he falls asleep in class for the third time in a week. Since the school cannot get a hold of Sophie, they end up calling Rebecca down to the school. She and Bret meet with an official from Child Protective Services, who fills her in on what has been happening with Martin. It is revealed that Rebecca has a rocky relationship with her mother after her father abandoned her a few years before (Paul wasn't her real dad, but her step-dad). Rebecca goes to talk to Martin alone. He tells her that he hasn't been sleeping lately. Bret drives Rebecca and Martin to Sophie's.Bret stays in the car as Rebecca takes Martin to the house. As they are walking, Martin tells her that Sophie has been talking to someone named Diana. Rebecca stops walking and asks Martin where he heard that name. He tells her that he heard Mom say it. Rebecca tells Martin that Diana isn't real. That she's someone their mom made up. Sophie opens the front door and welcomes Rebecca and Martin inside. When Sophie asks why Martin is home so early from school, he tells her that Rebecca will tell her why. Martin heads up to his room to pack some clothes (unbeknownst to Rebecca). Rebecca gets into an argument with her mother when she finds out that Sophie has been neglecting to take her anti-depressants. Rebecca tells Sophie that she's taking Martin over to her place for a few nights, just as Martin walks up with his suitcase all ready to go. Sophie begins to cry and pleads with Rebecca to let Martin stay, but Rebecca takes Martin and goes to her place.Later that night at Rebecca's place, she wakes up to find Martin gone from her bed. She hears a scratching noise and figures it must be Martin. She looks to where the noise is coming from and sees the figure hunched over and scratching the floor near the door of the bedroom. Since Rebecca lives above a tattoo parlor, the light of the sign above the tattoo parlor shines on and off every ten or so seconds. Rebecca can see the figure whenever the light of the sign is off and then it disappears when the light comes back on. The figure notices that Rebecca is watching it. So, it disappears and then reappears standing up in front of Rebecca and as it charges towards her, it disappears as the light comes back on. Rebecca races to the light switch, just as the light of the sign turns off, but she is able to flip on the light switch to her bedroom in time. She looks around for Martin and finds him in the bathroom, sleeping in the bathtub with a flashlight on.The next day, after taking Martin to school, Rebecca and Bret go to Sophie's. Since Sophie is out running errands, Rebecca and Bret go inside. Rebecca heads upstairs where she comes across a box with information on Sophie and her past stint in a mental institution when she was a young girl. When Sophie was in the institution, she met a girl named Diana, who had a rare skin disorder and could not go out into the light. The doctors tried an experimental procedure on Diana with a bunch of lights, which caused her to die. Rebecca hears a noise in her old room and when she goes to check it, the door to her room closes shut and she is yanked up to the ceiling by Diana, who tries to strangle Rebecca with her own necklace. Rebecca's screams are heard by Bret, who is able to get the door open and the light from the hallway causes Diana to disappear and Rebecca falls to the ground. Bret tells Rebecca that Sophie is coming home with Martin, so Rebecca grabs the box with the information on Diana and they both flee out the back door.Later that day, Sophie and Martin are spending some quality time watching an old movie. Sophie pauses the movie and tells Martin that she'll be right back. All of a sudden, Sophie goes and turns out the lights in the room. Martin gets afraid and Sophie tries to calm him down. She proceeds to tell him the story of her friendship with Diana. She tells him that Diana is her friend and can only stay if all the lights are turned off. In the background, you see Diana approaching behind Martin. When she appears beside Martin, he freaks out and begins to try to turn on some lights. Diana tries to stop him, but Sophie gets in her way and Diana smacks Sophie, sending her to the floor. This allows Martin time to get to the front door and escape.He goes to Rebecca's place. Bret goes to the store and gives Rebecca time to talk with Martin. Martin tells her that Diana is real. Rebecca acknowledges that he's telling the truth. Just then, a knock is heard at the door. Thinking it's Bret, Rebecca goes to answer the door and finds no one there. Rebecca and Martin begin to hear Diana in the walls. The same scratching sounds that Rebecca heard before begin to come from the closet in her bedroom. She slowly approaches the closet door and turns on the light. No one appears to be there. All of a sudden, Diana grabs Martin's leg and tries to pull him under Rebecca's bed, but Rebecca is able to pull Martin back out.With Bret, Rebecca and Martin go back to Sophie's. Rebecca tries to convince Sophie that Diana is dead, but that her spirit is trying to kill them, but Sophie ignores them and goes upstairs to her room to go to sleep. Rebecca tells Martin that all three of them are going to spend the night there. Bret will sleep on the couch and Rebecca will sleep in the same room with Martin. Rebecca goes to Sophie's bedroom to tell her that she's spending the night. As she walks away from the bedroom door, Sophie opens the door. She tells Rebecca that she's glad she's spending the night. Sophie grabs Rebecca's hand and slyly slips Rebecca a note. After Sophie closes the door, Rebecca looks at the note, which reads "I need help".Later that night, Diana cuts the power to the block. Bret goes outside with a flashlight to investigate as Rebecca goes to the basement (with a wind-up lantern) to try to find the fusebox.Martin wakes up and finds Rebecca gone. He grabs a candle, goes to the hallway and calls out for Rebecca. Behind Martin, Diana appears. He turns around and sees her in the dark. Almost immediately, she again appears behind him, this time the shock sends him to the floor. She begins to drag him away, but he turns around and shines the candle in her direction, this briefly sends her away. Martin makes his way downstairs to the basement, where he finds Rebecca. Rebecca figures out that it's a trap and they turn to go back upstairs. Diana appears at the top of the stairs and slams the basement door shut. They go to the basement door and begin to call out for Bret.Sophie begins to realize that Diana is not her friend. She calls out to Diana and tells her that she (Diana) cannot survive with her, since Diana as a spirit has latched herself onto Sophie. Sophie goes to take her medication, but Diana appears and smacks Sophie across the room. Sophie hits her head on a nightstand, knocking her unconscious. Bret comes back into the house and hears Rebecca and Martin calling out for help. He goes to the basement door and tries to open it. He turns and sees Diana down the hall in the kitchen. She begins to get closer to him. He shines the flashlight on her. She disappears for the moment but suddenly appears beside him, smacking the flashlight out of his hands and sending him to the floor. She begins to approach him, but he grabs his phone and uses the light to shine her away. With his phone in front of him, he uses it to keep her away, but as soon as the light turns off, she re-appears, smacking him again. He is able to make his way outside to his car. As he passes under the dark carport in front of his car, Diana grabs him. She lifts him up above her head, but before she can kill him, Bret grabs his car keys and hits the button for the car alarm. His car headlights turn on, sending Diana away and drops Bret to the ground, barely saving his life. He makes his way to the car and drives off to get help.Rebecca and Martin are still locked in the basement. She goes through a box and uses scrap pieces of paper to keep the fire in the furnace going. She leaves Martin by the furnace and looks for supplies that they might be able to use. Martin finds an old ultraviolet light that he gives to Rebecca. She looks through the basement and finds writing all over the walls of one section of the basement. Diana had been living in the basement because it's continually dark down there. Diana appears and Rebecca discovers that UV light cannot harm Diana, because it's not powerful enough.Bret arrives with two police officers. He stays outside by his car, while the two police officers head inside. They are able to get Rebecca and Martin out of the basement, but not before Diana kills one of the officers when he goes to check out the kitchen. Rebecca tries to warn the other officer that she needs to use a flashlight to stay alive. But, the officer doesn't listen. The officer sees Diana and fires her gun at her, but Diana disappears whenever the light from the gunshot appears, making a gun useless against her. Diana kills the second officer.Rebecca and Martin are heading out of the house, but Martin doesn't want to leave Sophie behind. Bret comes to the door and Rebecca tells him to keep Martin safe as she goes to get Sophie. Rebecca makes her way upstairs, but is attacked by Diana. Diana picks up Rebecca and throws her off the second floor balcony, where she falls hard on the ground floor, but she's relatively unharmed. Diana heads downstairs and is about to kill Rebecca, but Sophie appears behind her with a gun from one of the dead officers. Sophie tells Diana that if she (Diana) ever harmed any of her children, she would make her pay. Sophie puts the gun to her head and pulls the trigger, just as Diana is about to stop her from doing so. Diana disappears into dust as Sophie dies. Rebecca sobs for her mother.Later, Rebecca goes to the ambulance with Martin and Bret. Bret comforts the two of them as the power to the street comes back on. Fade to black.