Plot summary

Still struggling over the loss of their oldest son, Caleb, to the war in Afghanistan, the Peterson family Mr. and Mrs. Peterson and their children Luke and Anna are unexpectedly visited by David Collins, a former soldier who claims to have been a friend of Caleb's, and is there to fulfill his promise to Caleb to help take care of the family. David is polite, warm and friendly toward the family, and Mrs. Peterson offers to let him stay as long as he needs to.While staying with the family, David hears of Mr. Peterson's troubles at work, and witnesses Luke return home with a black eye. With Luke's help, he follows the students that attacked Luke to a bar, then humiliates them in a fight, using his knowledge of the law to convince the bartender not to tell anyone. David then goes to a party with a reluctant Anna, at which he saves her friend Kristen from her ex-boyfriend and has sex with her, also inquiring about where to buy guns from Anna's friend Craig. On their way home, Anna offers to make David a mix CD.David teaches Luke some self-defense and gives him his butterfly knife. When he goes to buy the guns from Craig and his friend, he kills them both before leaving with the guns, as well as two grenades. An increasingly suspicious Anna calls the military base to ask about David; this alerts a private corporation headed by Major Carver, who considers the matter of great importance and assembles a team of special forces before departing to find David. The base tells Anna that David died a week ago in a fire. Anna is distraught upon hearing that Craig has been killed and her boyfriend Zeke has been blamed for it, and asks Luke to do some research on the numbers David has called on his "burner" phone. It is also revealed that Mr. Peterson's boss and girlfriend are both dead as well, making him the new regional manager.At school, one of the students who David attacked at the bar calls Luke a "faggot"; Luke attacks him and both land in the principal's office. David arrives with Mrs. Peterson and convinces the principal to let Luke off with a month of after-school detention getting prepared for the homecoming dance. Luke tells David in confidence that Anna is suspicious and that he knows David must have killed everyone who has turned up dead in the last few days, but claims he doesn't care "'cause we're friends", and reveals that Anna may also have told Kristen about him. While David helps Mrs. Peterson with the laundry, Major Carver's team attack the house and attempt to kill David, who deftly kills everyone except Carver, and kills Mrs. Peterson after apologizing to her. While driving away, he crashes into Mr. Peterson's car, then kills him as well after apologizing again.Major Carver picks up Anna and informs her that her parents are dead and that David is really a former test subject for a military health program; his mental condition has been "programmed" to, if he believes his identity compromised, kill everyone who could possibly know about it. David kills Kristen and blows up the restaurant she works at with the grenades, then heads for the school to take care of Luke. Carver and Anna also head for the school, set up as a haunted house for the Halloween dance. When Carver and Anna attempt to help Luke escape, David announces his presence by playing Anna's mix CD, then killing Luke's supervising teacher and Carver when the four try to exit the building. Anna shoots David with Carver's gun, but David stabs her in the leg and attempts to choke her; Luke rescues her by stabbing David with his own butterfly knife twice. As he seemingly dies, David tells Luke that he did the right thing and gives him a thumbs-up.While sitting in an ambulance outside the now-burning school, Anna and Luke overhear the conversation between the firefighters: they found only two corpses with their teeth pulled off. Anna witness a firefighter enter the building and then leave; the leaving firefighter appears to Anna to be David, who has seemingly survived.