The movie starts off with Grady Edwards (Dylan Walsh) in the bathroom looking in the steamed up bathroom/mirror, as he puts some dye into his head and rubs it in. It later shows him showering, and then as he wipes the mirror off, he cuts off the hair on his beard and shaves. He then removes colored contact lenses from his eyes.
He makes his way by grabbing his things downstairs, as it shows family pictures of him with the children, and his wife. He turns on the radio to play Silent Night, and you can see by the decorations around, and the song that it's around Christmas time, or even on Christmas day, morning preferably or the day afterwards. He goes into the kitchen where he puts toast into the toaster, and grabs some coffee. He spreads some peanut butter on his toast, and as he's eating he walks around the house a bit, and as he does a little boy is seen with his head down in a sleeping position at the breakfast table.
He also looks into the decorated living room, to where it also shows his wife in a lying down position, and another child near the Christmas tree in a lying down position. He picks up his bag and heads for the door, as a flashback comes in with a girl running to the door screaming. At this point as he is about to exit, and punches in the security code there is a little girl lying on the floor. He leaves and gets into his car.
It is shown that the bodies of the woman, and her three children have been found, and the father of the children was devastated upon learning about his children's death, and especially in that way and that their stepfather is nowhere to be found.
As this is being played out, the stepfather is seen as he goes from one place to another, changing cars, and places and he heads into a town, in where he is at a grocery store. He's in the cookie section, and he overhears a woman (Sela Ward) who introduces herself as Susan Harding with her two children (Braeden Lemasters) Sean Harding, (Skyler Samuels) Beth Harding arguing about what cookies to get. He sides with the woman, and says that women rule and they know best especially when it comes to cookies. That gets a smile from the woman. The stepfather introduces himself as Daniel Harris. She sees his wedding ring and asks him about his children, and he explains how he lost his wife and daughter in a car accident last year. She apologizes profusely and he accepts it and they depart, even though as they go through the check out line aisle they check each other out. She also fixes her hair a bit. Later on she invites him out for pizza with her kids and as he gracefully accepts.
Six months later. A young teenage girl (Amber Heard) named Kelly Porter is picking up her boyfriend (Penn Badgley) Michael Harding from somewhere, and as they're in the car talking, she's going on about how he's a great guy, and he's not all that impressed.
They make their way into the house and as he is looking around in the house, the girl, Kelly, goes out into the yard to which she calls the boy, Michael out to join her. He does, and is surprised by his entire family, and friends to welcome him home. As he sees everyone again, and meets the man that his mom has been seeing for quite some time, is now living with, and is soon to be married to, David Harris.
They talk a bit, and Michael seems suspicious of him even upon first meeting him and is not sure what he thinks of him. Despite David going out of his way to impress him.
Michael looks in the basement and sees all these cabinets in there with locks on them. He touches them and is told in a very firm voice by David, that those are his, yet Michael can take any other ones, and David reveals that he's a carpenter wannabe and is experimenting with building things.
David tells Michael he heard Michael is a swimmer and says he use to swim back in his college at the U of O back in the day. They share a drink, of some stash of alcohol that David has hidden.
Time passes, and David is told that Michael likes to swim a lot, and is a good swimmer so he convinces the swim team coach to let Michael back on the team on a probationary basis, and as he heads home he hands Michael the key to his locker room. As Michael smiles, about this you can see he might be opening up to him just a bit.
A nosy old woman with a 'gazillion' cats named Mrs. Cutter shows up at the door and is greeted by Susan. She and Susan talk about David, as Michael overhears. Susan dismisses her nosy neighbor and heads into the kitchen where she shares with David Mrs. Cutter's suspicion/accusation that he's that stepfather that America's Most Wanted is looking for, since he fits the description. This ticks off David, yet he doesn't let it show and asks Susan just how reliable this woman is. To which Susan replies that she knew about someone's affair, and told everyone else before telling him, and she was right. David leaves for work and as he is driving by, he waves at Mrs. Cutter as he leaves which spooks her out. Michael heads downstairs and he and his mom talk as she prepares him breakfast.
David is at lunch with Michael and he gets the name of his child wrong, first he said it was Michelle then said it was Lisa, and how she'll be 11 next week or would have been, and then Michael calls him on it, to which David replies her name was Lisa Michelle that's why and he gets angry about it, so much so that when he puts an open house sign in, he's hammering it in totally oblivious to all that is around him. He glances up when he sees a couple that is interested in the house and they make their way inside.
David greets Mrs. Cutter upon coming home, and later on her doorbell rings and as she goes to answer it, there's no one there. As she goes back inside, she sees her cat acting wildly and goes to see what's wrong. As she does, the cat shrieks loudly, and starts hissing and she wonders why, as she gets up, she sees David and screams, and he hits her and drags her across the room and he throws her down the basement stairs. He holds her cat and heads down the stairs and holds her mouth so she can't breathe until she dies. He leaves.
He heads back to the house, and Michael and Kelly are hanging out by the pool and Sean is playing video games with the music blasting. His mom Susan asks him to turn it down twice, yet he keeps playing it real loud. David comes in angrily into Sean's room and grabs him violently by the neck and tells him, "Your mother asked you to turn it down." Sean is stunned, shocked and scared not sure what to do about what just happened. He's close to tears as well but doesn't say anything.
At dinner time he barely says a word, and the doorbell rings and it's their dad, (Jon Tenney) Jay Harding, and he greets his kids as they head to the car and talks to Susan about his arrangement with the kids and they argue a bit. He leaves.
Susan comes back in and mentions to David about how her ex is such an ass/jerk.
Susan's ex shows up and punches David out, and yells at him for hurting Sean, and Michael takes his father outside, while Susan comforts her son, and David apologizes to him for hurting him, and says he made a mistake, Susan makes it clear what he did was unacceptable. He tells her she's right.
Michael is still suspicious of David as he snoops around the house. David is at work and (Paige Turco) Jackie Kerns, tells him that he needs to file out his paperwork so she can stop paying him under the table and he can get paid. She needs his photo I.D. He tells her he'll take care of it right away and puts the file into his desk. Throughout the next few days or weeks she keeps trying to contact him about this, and he keeps telling her he'll get to it, and ignores it. He tells Susan he wants to work somewhere else, as it's not working out for him in real estate, despite being a natural at selling houses.
He tells Susan he'll look online for places he could work that require less travel and goes to use her computer. Susan tells him that she's going out with her friends, Jackie and Jackie's friend (Sherry Stringfield) Leah, and they go out for drinks. While Susan is out with her friends they tell her about their suspicion about David's avoidance. And Michael and Sean are playing video games. Sean is still fearful of what David might do about the noise in the game, yet Michael tells him it's cool.
Jay shows up at the door and apologizes to David for his behavior and tells him he's heading to the airport and wants to say goodbye to his kids. David invites him in, and goes into the kitchen to answer the phone. It's Susan who asks him if Jay showed up, as he said he would to say goodbye to the kids. David says no, he's not there when he is and hangs up with her. Jay talks to David about his suspicions of him, and about his lie about him going to the U of O College and his name and wants to know what else he's lying about. He goes into the kitchen to find it empty, and wonders where David went. He turns around as David smashes Jay's face in with a crystal vase and Jay crashes to the floor.
Michael and Sean hear the commotion and stop playing their game. David holds Jay down, then as he hears them go back to their game, drags Jay down the stairs, as he attempts to suffocate Jay by putting a plastic bag over his head. Michael heads downstairs to see what the noise is about again and Jay is struggling but dies. David then goes and tells the taxi driver that Jay changed his mind and pays him off and unloads all his stuff out of the car and puts it into the locked cabinet in the basement. David sends Michael a message from Jay's phone saying he still doesn't like David but at least he 'seems to be who he says he is'. Michael then becomes tearful.
The next day, and Michael looks out the window as David is talking to the mailman about police cars across the street. He finds out that Mrs. Cutter died by falling down the basement stairs and breaking her neck. David tells Susan and the kids about that, and they're shocked about it. Susan feels bad for saying horrible things about Mrs. Cutter and David just shrugs like no big deal. Susan tells David she's heading out and tells him not to forget to pick up the kids from the dentist. He tells her ok. As he does, he heads up to the attic to work on something, and gets a phone call. It's Jackie who tells him again that she wants that paperwork done, and he tells her he's not working there anymore and doesn't think it's necessary. She tells him it is necessary and she needs them ASAP. He tells her he will when he has time, yet he has to pick up the kids from the dentist, and is making up a lot of excuses to avoid it. He hangs up on her, and Michael tells David about Mrs. Cutter's tragedy about how it's a shame and David says accidents happen.
Michael and Kelly are in the back in the pool area trying to fill out some college applications and he's talking about his suspicions again, and they go into the pool to swim and make out. David shows up and tells Michael that he's going to go pick up the kids and tells them to behave and leaves.
At this point Michael and Kelly make their way into the house and start to snoop around, and David gets a call from Susan who tells him she's picking them up as she's closer and David heads back to the house. He sees that Michael and Kelly are no longer in the pool, or in the yard and goes into the house looking for them as he sees their footprints on the ground. He looks into Michael's room and doesn't see anything, and goes into his room, he looks outside again and sees they're still not in the yard. And as he heads in his room, he hears a splash and some laughing and sees Kelly and Michael back in the pool and then he shuts the curtain.
Michael also checks on the computer and sees the America's Most Wanted site up in the search and sees the article about the stepfather and the slayings and prints out the photo for Kelly to see to which she thinks he's reading too much into, and it's probably nothing. Michael takes a photo of his mom and David, and later on takes a look at the photo he has, and the photo of the man that is wanted.
That night, Susan takes her two younger kids to some place so they can go camping for a couple of days. She'll be gone at least a couple of hours, and David tells her he'll just search for jobs, and maybe jog. He goes to see what Michael is up to, and Michael says he's heading to see Kelly. Michael sees that his photo on his phone OF David is gone, and knows it was David that deleted it and goes to see Kelly and tell her about it.
Jackie is still suspicious of David and tells her girlfriend Leah about it, and tells her she can't wait to see her, and is heading out. She tells Leah that she left a voice message for Jay, and hopes that Susan won't be mad at her, but will leave her a message too about her suspicion and David's avoidance about this paperwork. She sends her an email, which David sees, reads, and deletes. Then he heads over to Jackie's place and drowns her in her pool.
Then he heads home, and Susan comes home, and sees David and he tells her he was out jogging to clear his head, and he tells her that Michael is at Kelly's. They head upstairs, and David gives her a sleeping pill to relax her, and she takes half of it. They head to bed. As she is sound asleep, he's wide-awake looking up at the ceiling, and it is shown in the bathroom his tools (killing tools maybe?) laid out waiting for a good time.
Michael heads home with Kelly in tow in the car and he tells her of his suspicion, and tells her he's going to check things out in the basement and if she sees any light on in the house to warn him, as he has his phone on vibrate. He heads into the yard and breaks the window to get into the basement. He slips in and breaks the lock and sees a bag, he empties it out and sees the freezer. As he heads to it, Kelly sees the light has opened in her house and heads over to warn him by the phone. His phone vibrates and it falls, breaking, and he goes to pick it up and the pieces. Then he looks in the freezer and sees the dead body of his father in the freezer and screams.
Kelly heads out to warn Michael and is knocked out by David, who drags her back into the house and is in the kitchen taking out the duct tape, and some knives and screaming how it's not working out and he needs a new family and blah blah. Susan enters and asks him what's going on, and why is he shouting. He tells her he can't take it anymore and wanted her to be Mrs. Grady Edwards, to which she asks him what he said. He realizes his error too late and says "Who am I here? I meant David Harris", and she screams as she sees him grab a knife and chase her through the house. She's screaming and Michael is down in the basement screaming to get out, and he does and grabs Kelly and she sobs and they head up. Susan heads into the bathroom and slashes David in the throat with a piece of glass that David throws at her. It looks like he is dead as he falls into the shower and she heads out, backing away and embracing Michael and Kelly.
Then David shows up and Michael and Kelly and Susan head up to the attic, David chases them up, and they're screaming for help, and he tells them the boards are loose and Kelly grabs a jigsaw and tries to throw it on David, and Michael is screaming no and falls through the crack board, and then Kelly and Susan fall through the roof, and David looks down on them. Then Michael grabs David and they go through onto the roof outside, and both of them topple over onto the ground.
Michael is in the hospital as his mom and Kelly are there comforting each other, and he's in a coma waking up.... A month has passed, and they explain by the time the police got there David disappeared and he's gone. And as he's taking all the news in, Michael is in shock and it changes to another scene, in a store, a woman and her two boys. She chases her two boys somewhere, and she meets a man.
The Stepfather now has a bandage on his neck, and he greets her, and tells her his name which is now Chris Ames, and smiles. The woman introduces herself as Julie King and smiles back. And it looks like the Stepfather is starting again with his charms as he targets a new woman as it all ends (Another one of those annoying here-we-go-again twist endings).