Katie, a student, and her boyfriend Micah, a day trader, live in a two-story tract house in suburban San Diego, California. Katie claims that a ghostly presence has haunted her since her youth and believes that it has followed her to their new home. During a visit to their house, Dr. Fredrichs, a self-professed psychic, assesses that they are being plagued by a demon which feeds off negative energy and is intent on haunting Katie no matter where she goes. Dr. Fredrichs recommends they contact Dr. Johann Averies, a demonologist, but Micah's reluctance to do so leads to arguments with Katie. Each night, Micah mounts a video camera on a tripod in their bedroom to record any paranormal activity that might occur while they sleep. He manages to capture several phenomena, such as objects moving on their own, lights and televisions switching on and off, and sounds ranging from voices and growls to footsteps and loud thuds. The strange occurrences in the middle of the night soon involve Katie awakening to spend several hours standing by the bed staring at Micah while he sleeps and going outside to sit on the backyard swing, none of which she remembers the following morning.
Katie, already irritated by Micah's making light of the situation, becomes irate when Micah brings home a Ouija board, despite Dr. Fredrichs' warning against any attempt to communicate with the entity. While the two are out of the house, a wind blows through the living room and the Ouija board's planchette moves on its own. A small fire soon erupts on the board, extinguishing itself seconds later. That night, Micah sprinkles talcum powder in the hallway and later they find inhuman footprints leading to the attic. There, Micah finds a charred photograph of a young Katie, previously thought to have been destroyed in a house fire. That night, the bedroom door slams shut and is violently banged upon from the other side. The next day, they discover the glass over a photo of them has been smashed and Micah's image gouged with a deep scratch. Dr. Averies is abroad when Micah finally agrees to invite him, so Dr. Fredrichs agrees to make a second house call. However, Dr. Fredrichs immediately has a sense of foreboding upon entering their home, and apologetically leaves despite their pleas for his help.
That night, Katie is dragged out of bed and down the hallway by an invisible force. Hearing her screams, Micah gives chase and wrests her away; the next day he discovers a gruesome bite mark on her back. Stressed and exhausted, the couple decide to go to a hotel. Micah later finds Katie gripping a crucifix so tightly that it bloodies her palm. Just as Micah is set to leave, a suddenly calm Katie insists they instead remain at the house. That night, Katie awakens to once again stare for several hours at Micah while he sleeps. She then goes downstairs into the darkness and begins screaming. Micah wakes up and runs after her, while the camera, left on its tripod, records what sounds like a struggle downstairs. The screams suddenly stop, and a brief silence is followed by the sound of heavy footsteps coming up the stairs. Micah's body is violently hurled at the camera, knocking it over. Katie slowly walks into view, her clothing soaked with blood. She crouches over Micah's body, eerily smiles at the camera, then lunges toward it with a demonic growl as the screen cuts to black. An ending title card states that Micah's body was discovered days later by the police, and Katie's whereabouts remain unknown.
