Micah, a day-trader, and Katie, a college student are a young couple "engaged to be engaged". Katie claims that a ghostly entity has haunted her since her youth and believes the ghost has followed her to their two-story tract house in suburban San Diego, California. Micah buys a video camera in the hopes of capturing paranormal activity on film, a project that will ultimately last three weeks. 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 also recommends the two contact Dr. Abrams, a demonologist, and Micah's reluctance to do so leads to frequent arguments between the couple. Each night, Micah leaves the video camera on a tripod in their bedroom to record whatever might be occurring while he and Katie sleep. He manages to document 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 and staring at a sleeping Micah, and going outside to sit on the backyard swing — behavior which she does not remember the following morning.
Katie, already irritated by Micah's making light of the situation, becomes further infuriated when Micah brings home a Ouija board, despite Dr. Fredrichs strongly discouraging 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 before a small fire erupts and extinguishes on the board. Later, inhuman footprints are left in baby powder placed on the hallway floor by Micah. In the attic, Micah finds a charred photograph of a young Katie previously thought to have been destroyed completely in a house fire. The next night, the bedroom door slams itself shut and is violently knocked and banged upon from the other side. Dr. Abrams 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 Katie and Micah's desperate pleas for him to stay and help.
That night, Katie is forcibly dragged out of bed and down the hallway by an invisible force. Hearing her screams, Micah gives chase and wrestles her away, and Katie emerges from the struggle with a gruesome bite mark on her back. Stressed and exhausted, the couple decide to go to a hotel room. That day, a glass picture frame breaks and the portrait inside becomes scratched. 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. When Micah leaves her, she begins to smile at the camera as she closes her eyes. That night, Katie awakens to once again stare at a sleeping Micah for several hours. She then goes down the stairs into the darkness and begins screaming. Micah wakes up and runs to her aid, and the camera can only see the bedroom and dark hallway as it records what sounds like a struggle upon his arrival downstairs. Micah and Katie's screams suddenly stop, and the brief silence is followed by the sound of heavy footsteps coming up the stairs. Micah is suddenly hurled at the camera, knocking it over. Katie slowly walks into view, her clothing covered in blood. She crouches to sniff at Micah's lifeless body, then smiles at the camera before lunging toward it 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.
