After experiencing the devastating loss of their eldest son Caleb in the Afghanistan war, the Peterson family—consisting of the father Spencer Peterson, mother Laura Peterson, and their children Luke and Anna—find themselves unexpectedly visited by David Collins, who claims to be Caleb's former soldier comrade and best friend. He presents himself as the bearer of both good intentions and past memories, asserting that he was by Caleb’s side when he perished and that Caleb explicitly asked him to keep an eye on his family. David even goes as far as identifying himself in a family photograph taken in Afghanistan.
Expressing his warmth and friendliness, David reveals that he has recently returned from a long hospital stay due to an injury sustained in service, and he plans to lodge at a nearby motel. However, Laura, out of kindness, opens the doors of their home to him for as long as necessary. Meanwhile, Anna is seen grappling with troublesome patrons at her late-night diner job, while Luke is victimized by bullying jocks at school. When David learns of these troubles, he quickly steps in to assist Luke with his math homework, and their bond begins to grow.
The next day, David channels his protective instincts toward Luke by confronting the bullies at a bar, which culminates in a heated altercation that leaves them injured. Demonstrating his impressive knowledge of the law, David manages to convince the bartender to keep the incident under wraps. At a party, he unexpectedly comes to the rescue of Kristen, a friend of Anna's, from an aggressive ex-boyfriend. The night takes a more intimate turn when Kristen ends up spending the night with David, while he expresses an interest in obtaining firearms during a conversation with Anna's friend Craig.
As David begins to take on a more active role in the family's life, he gifts Luke a butterfly knife and teaches him self-defense techniques. However, behind the facade of helpfulness lies deeper, darker intentions. Anna grows suspicious of David's true identity and makes a fateful call to the military, discovering that the real David Collins died in a fiery explosion at a military hospital—where the falsely identified David had claimed to be recovering.
As tensions rise in the house, a shadowy special forces team led by Major Carver tracks David to the Peterson home. In the resulting chaos, David's true nature is revealed; he ruthlessly murders both Laura and Spencer in an effort to cover his tracks. Carver informs Anna of her parents' deaths and unravels the truth about David: a failed military experiment turned killer.
The story spirals into a heart-pounding conclusion. David targets the school to eliminate witnesses, while a Halloween dance becomes the backdrop for an explosive confrontation. With the stakes apocalyptically high, a chilling sequence of events unfolds as Anna and Luke band together for survival. When it seems like the nightmare is finally over, David's sinister presence looms large, leaving Anna and Luke to question how far they will go to escape his grasp and whether they can truly be free from the horrors he represents.
