As phroggers—individuals who stealthily occupy others' homes—teenagers Mindy and Alec invade the residence of police detective Greg Harper, his wife Jackie, and their teenage son Connor. Despite Mindy's insistence on remaining unnoticed, Alec begins to torment the family, engaging in bizarre acts like removing pictures from frames, turning on TVs, and even urinating on Greg while he sleeps. Adding to the chaos, he conceals Jackie's beloved sunflower mug, leading the family to ponder whether a supernatural force might be at play within their home.
Tension simmers between Connor and Greg due to a recent revelation about Jackie's affair, causing Greg to act out in frustration, even smashing his phone against a window. When a repairman appears to fix the damaged window, Alec deceives Mindy into letting him inside, not realizing she doesn't actually belong there. Jackie's surprise grows as she returns home to find this stranger within her house. Upon discovering stolen silverware, which Alec cleverly hid, Jackie turns to the police for help.
Lieutenant Moriah Davis assigns Greg and his partner, Ray Spitzky, to look into the mysterious disappearances surrounding two boys, including 12-year-old Justin Whitter, who vanished while riding his bike. At the crime scene, they uncover a green pocketknife reminiscent of one owned by Cole Gordon, a man incarcerated for murdering six boys long ago.
Greg and Spitzky suspect they may be dealing with a copycat killer and decide to interview Gordon's surviving victims. They visit Tommy Braun, whose horrific disfigurement from a previous attack becomes chillingly evident during their conversation. Tommy cryptically recounts a "magic house" that can disappear, warning Spitzky that he will never find the man responsible.
Meanwhile, Jackie's lover, Todd, visits the Harper home to express his affection, only to come under attack when Alec hurls Jackie's precious mug at him from above. Mistakenly believing her son, Connor, is responsible for the incident, Jackie tends to Todd's injury in the basement while Mindy, anxious to escape, fetches her concealed backpack. However, she is caught in a life-threatening situation when Greg unexpectedly returns home and discovers Todd in the basement. In a shocking turn of events, Mindy witnesses Greg murder Todd in a brutal attack.
As the plot thickens, Jackie and Greg conspire to bury Todd's body in the woods, while Connor stumbles upon the hidden silverware and receives a sinister anonymous message about "phrogging." Before long, Alec confronts and captures Connor, leading to a turbulent conflict that leaves Mindy unconscious in the back of Greg's vehicle.
Upon waking, Mindy discovers a horrifying reality, finding evidence that indicates Greg's sinister past, including the presence of Justin Whittier's jersey. In a perilous escape, she manages to flee into the woods and contacts the authorities. However, before they can trace her location, the call abruptly disconnects.
Finding a secret trailer where the missing boys are imprisoned, Mindy learns of Greg's horrific intentions. Just as she attempts to rescue them, Greg catches her and suffocates her, believing her to be a threat. In a heated and frantic series of events, Greg shoots Mindy, making it appear as though she was attacking him in self-defense and plants a weapon in her hand to cover his tracks.
In a final confrontation, Alec and Greg face off. After a duel involving an axe, the scene culminates in Greg's demise, shot by law enforcement right after he learns Alec's intention to expose him. The narrative wraps up with Alec whispering to Spitzky his name, revealing his connection to Tommy and a vengeful plan that unraveled in tragic ways, while the police rescue the two boys hidden in the woods.
In a poignant ending, Jackie and Connor return home, greeted by a swarm of police activity and the harsh truth of Greg’s hidden life. As memories resurface, Alec recalls a moment from his childhood—the simplicity of walking along a railroad track with Tommy, a stark contrast to the nightmarish revelations that just unfolded.
