The film opens with a notice, revealing that a young man named Thomas was convicted of raping and killing his sister, and murdering Dr. Edgar Lauren, a psychic. Despite his pleas, he was sent to prison, where he committed suicide.
One night the camera records Ellen sitting up in bed and leaving her bedroom. She walks into the living room, where she writes something on a piece of paper. She then crumples the paper and walks back down the hallway. Thomas finds the paper under Samantha's pillow and sees that it spells out the word MARON.
Thomas sets up traps around the house with three bells and thin wire. Should the entity enter a room, the bell on the wire will jingle. When this eventually occurs, the wire and bell outside his door are ripped from the wall and thrown at him. Thomas shuts himself in the bedroom and the demon bangs on his door.
Thomas receives a phone call from Ellen, who is in hysterics after the demon apparently followed them and attacked Samantha. Thomas awakens at night and finds his sister missing, with the front door open. He searches outside but doesn't find her. He finds the attic ladder hanging down and ascends it to find her standing there undressed, in a trance.
Thomas hears Samantha screaming and rushes to the bathroom. He finds her lying topless in the bathtub, severely traumatized.
Ellen wakes after hearing thuds from outside. In the living room camera, she is seen standing in the archway with a knife before returning to her bedroom. Thomas wakes up after hearing a door slam shut, and finds that Ellen has slit her wrists, weapon still in hand. She is taken to the hospital.
Thomas gives Dr. Lauren the note that says "MARON". Dr. Lauren explains that "maron" is Old Germanic for "nightmare", a creature similar to the incubus, a demon that rapes women in their sleep. The psychic agrees to help them evict the entity and the video fades to black.
After a pause, the camera's POV shows the doctor's bleeding head and blank face, fallen on the floor and "looking" toward the lens. Thomas is heard panicking.
Thomas grabs the camera after hearing Samantha's scream and runs to her bedroom. We see that the house is in total disarray. Thomas finds his sister naked and levitating above the floor in her room, covered in blood, while being raped by an invisible demon. He drops the camera and tries to help her. A gurgling noise is heard offscreen, and an unseen figure (heavy breathing can be heard), picks up the camera and focuses it on Samantha's lifeless face and breasts.