The film opens with a title card stating that in 2006, Katie Featherston and her boyfriend Micah Sloat moved into a suburban house in San Diego, California. Strange events begin occurring in their home, so Micah sets up a camera in their bedroom to try to film any paranormal activity.
The first night, the camera records noises of doors slamming shut and footsteps in the hallway while Katie and Micah sleep. Micah brings in a Ouija board, which angers Katie. They attempt to communicate with any spirit present, and the planchette moves to the letters that spell out the name “Diane”. Katie abruptly ends the session and goes outside alone.
Over the next few nights, more noises and moving objects are caught on camera. One night the bedroom door slams shut by itself, startling them awake. Katie claims she has had a demonic presence following her since childhood. Micah remains skeptical but concerned for Katie’s well-being.
Katie calls a demonologist named Dr. Fredrichs and leaves a voice message asking for his help. As paranormal occurrences continue, Katie appears to fall into a trance and stand motionless over Micah while he sleeps. Micah finds the next morning that she has no memory of the incident.
Micah discovers Katie’s picture from when she was a child torn up in the hallway. He replaces their bedroom door with a heavier one after the original keeps opening by itself at night. One night the sound of loud, rapid footsteps come from the hallway, terrifying them.
Katie insists something has bitten her thigh, but Micah cannot find any marks on her skin. The next night, Katie wakes up and stares at Micah silently again for two hours before returning to bed with no recollection of it in the morning.
Fed up, Micah sprinkles baby powder in the hallway and outside their bedroom to detect the presence. The powder shows non-human footprints leading to the bedroom and markings on the bed sheets by Katie’s legs. Katie calls Dr. Fredrichs again, pleading for help.
Katie appears ill and says she feels something evil growing inside of her. Micah finds her standing motionless over him again that night. In increasing daylight, a loud rumbling comes from downstairs as if someone is moving furniture. Katie’s screaming is heard as Micah rushes to intervene. He finds her screaming and crying downstairs. That night, Micah finds the photograph of young Katie torn up in the hallway again.
Katie insists they call the demonologist. As Micah is out of the house picking up Dr. Fredrichs’ number, a possessed Katie screams and runs around, slams herself against the bedroom door, and whimpers Micah’s name before everything goes silent. A worried Micah returns to discover blood on the bedroom door and Katie’s body thrown against the camera, apparently dead. Micah panics, grabs a knife, and heads downstairs. Suddenly, Katie’s body is violently dragged down the stairs by an unseen force.
Micah screams as he is thrown at the camera, dropping the knife. The possessed Katie stands in the doorway, her clothing and hands covered in blood. She slowly walks into the bedroom, crawls to Micah’s body, and proceeds to bite and rip at his flesh as the screen fades to black, leaving his screams lingering over the end credits.