A mysterious stalker records video of the married couple James Nick Stahl and Amy Mia Kirshner Deakin as they hide their spare key outside their home. When the couple leaves to jog, he uses the key to enter their house and install hidden security cameras in each room. He also makes subtle changes, such as setting their alarm to go off earlier and leaving a mix CD in their car. Amy protests when the alarm wakes her up early, and the two argue over who recorded the mix CD. When James finds the audio tracks on his computer, Amy considers the mystery solved, but James insists that he never downloaded them. As they argue, James dismisses Amy's PhD research on Afghan culture as a waste of time, and she refuses to speak to him. The next morning, as James leaves for work, he writes her a brief apology. The stalker enters the house, reads the note, and records video of Amy's reaction to noise that she hears in the house.
When James returns home, he finds a note ostensibly written in Amy's handwriting in which she says that she has left to clear her head. James attempts to contact Amy many times, but she does not answer her calls. Worried, he calls her friends, including Amy's sister Katherine Krista Bridges, with whom he has an antagonistic relationship. James refuses to explain the situation to Katherine, and she becomes suspicious that something has happened between Amy and James. James eventually turns to his friend Alex Aaron Abrams, who suggests that their childhood acquaintance Bill Devon Sawa may be involved. James contacts Bill, an Afghanistan veteran who now works nights at an animal shelter, and he apologizes for tormenting Bill mercilessly during their school years. Although Bill accepts both the apology and a gift, a collectable baseball, he says little except that the apology does not negate the bullying.
Later, James comes to believe that his cat has been replaced when he receives taunting e-mails and the cat exhibits uncharacteristic behavior. A police officer files a report but does not take James' claims seriously. When James finds the remains of his decapitated cat and video surveillance evidence that an intruder entered his home, he contacts the police again, but the police remain skeptical, as the cat's remains have gone missing. Convinced that the police will not act, James investigates the matter himself. James receives a brief video that shows Amy bound and gagged before it deletes itself. Frustrated, James demands the unseen antagonist reveal himself; immediately afterward, the collectable baseball is thrown through one of his windows, which initiates a violent confrontation between James and Bill. Hidden video surveillance later depicts James as he digs a shallow grave.
James becomes erratic and paranoid, and, with Alex's help, he illegally purchases a handgun. Katherine becomes increasingly suspicious of James and threatens to call the police, which James laughs off. James attempts to make contact with his unseen stalker, who intentionally reveals himself as being in the house. After a brief chase, James offers a deal: for Amy's safe return, James will stop investigating and not further contact the police. At work the next day, James receives video footage of Amy asleep in their bed. James leaves for his house at the same time that Alex, Katherine, and a police officer arrive. As Katherine discovers Amy's body hidden in the basement, James attempts to confront the stalker, only to realize that he has been framed for her murder. While still holding the pistol, James is shot dead by the police officer. In his home, the still-unidentified stalker labels his surveillance tape as "388 Arletta Avenue" and sets it beside several other tapes. He then starts the cycle again with a new family.
