Plot
In a suburban Utah house, Grady Edwards shaves off his beard, dyes his hair, and removes his brown contact lenses. As he leaves, it is revealed that he has murdered his wife and three children. When the police investigate, it is said that another family in New Jersey was murdered in a similar manner not long ago, which causes them to believe there is a serial killer on the loose.
Susan Harding, a recently divorced Oregon housewife, is shopping in a grocery store with her youngest children when she meets Grady, who introduces himself as "David Harris", and claims that his wife and daughter were killed in a car accident. Susan, desperate for love, is charmed by David, and rushes into an engagement to be married in less than six months.
Susan's eldest son, Michael, returns home from military school and is immediately suspicious of his mother's new fiancé. David invites him down to the basement, where he has installed locked cabinets and befriends him over tequila shots.
However, Michael's suspicions start when David uses the wrong name when mentioning his deceased daughter. After Susan says that a neighbor warned her that America's Most Wanted ran a profile on a serial killer who looks like David, he sneaks into the neighbor's house, throws her down the basement stairs, then suffocates her.
Jay, Susan's ex-husband, angrily confronts David over laying hands on his younger son Sean. He warns Susan that she knows nothing about him. Doubts about David mount further when he quits his job working as a real estate agent for Susan's sister, Jackie, to avoid displaying a photo ID and other forms of identification.
Later, Jay confronts David about an apparent lie regarding his college history. David clubs him with a vase and suffocates him with a plastic bag. He texts Michael with Jay's phone saying that David checked out okay.
When the neighbor's body is discovered two weeks later, David tells the family. Michael is alarmed because he overheard David being told by the mailman, who gave less detail than David tells them. While Michael's girlfriend Kelly tries to get him to focus on college applications, he grows more obsessed with the contradictions in David's stories. The situation comes to a head when David intercepts an email from Jackie about hiring an investigator. He then goes to her house and drowns her in her pool.
Determined to discover what is in the locked cabinets, Michael breaks into the basement as Kelly keeps a lookout. Michael eventually discovers his dad's body in a freezer. David knocks out Kelly and traps Michael in the basement.
The commotion awakens Susan, and David berates her parenting skills, saying that he thought she could be "Mrs. Grady Edwards". On Susan's stunned reaction, he grimaces and asks, "Who am I here?" Susan tries to snap him out of it, causing him to say, "David! I'm David Harris!"
Susan, realizing the situation after noticing the unconscious Kelly, flees to the bathroom, locking herself in. David kicks the door in, shattering the mirror behind it. Susan picks up a shard of the mirror, holding it behind her. David grabs her, they struggle, and she manages to stab him in the neck with the shard. He falls into the bathtub, apparently dead.
Michael escapes from the basement and finds Kelly. They find Susan in the hallway across from the bathroom, thinking David is dead. Then David approaches from behind and blocks the stairs, chasing all of them into the attic, where he and Michael fight. Both fall onto the roof and then off its edge to the ground, where they lie unconscious.
When Michael wakes up, he finds out he has been in a coma for just over a month. He learns that David is still alive and fled the scene before the police arrived. The end scene shows David, who has again changed his appearance and his name to Chris Ames. He is working at a hardware store when he meets a woman who is shopping with her two sons.
