High school senior Billy Lynch lives with his overprotective aunt, Cheryl, who has raised him since the tragic loss of his parents in a car accident. As a talented basketball player, Billy receives an exciting scholarship opportunity to the University of Denver. However, Cheryl dismisses this chance, believing Billy should remain by her side to “contribute” to the household. His struggles continue at school as he faces bullying from teammate Eddie, who feels threatened by the close bond Billy shares with their coach, Tom Landers.
On the eve of Billy's 17th birthday, a pivotal moment occurs when Cheryl changes her mind about the scholarship. She requests that Billy stop at the television repair shop to have technician Phil Brody take a look at their faulty TV. That night, after fixing the television, an unsettling turn of events transpires when Cheryl makes unwanted advances towards Phil. When he declines, she brutally stabs him to death with a kitchen knife, a horrifying scene witnessed by Billy through the window. In a twisted lie, Cheryl claims that Phil attempted to rape her.
Enter Detective Joe Carlson, a former Marine and Purple Heart recipient, who takes on the case. His instincts lead him to be skeptical of Cheryl's claims regarding the alleged assault. Unraveling the truth, he discovers Phil's sexuality and his relationship with Coach Tom, suspecting a love triangle entangled with Billy and that Cheryl is protecting her nephew. Carlson's interrogation of Billy becomes increasingly hostile as he labels him with derogatory terms and harasses Tom, leading to Tom's resignation from the high school coaching position. He also probes Julia, the school newspaper photographer, about her connection to Billy. In the midst of the turmoil, Cheryl starts feeding Billy drug-laced milk, sabotaging his performance at his scholarship tryout, while also converting the attic into a space for him to live in.
After witnessing an intimate moment between Billy and Julia, Cheryl's rage boils over. In the attic, Billy stumbles upon a photo of Chuck, said to be one of his mother’s past boyfriends. He enlists Julia’s help to distract Cheryl while he investigates further; rummaging through a locked box, he uncovers his birth certificate, revealing that Cheryl is indeed his mother, and Chuck is his father. Meanwhile, Cheryl violently attacks Julia with a meat tenderizer and continues to drug Billy, leaving him unconscious.
When Julia regains consciousness, she finds herself in a secret basement room. Here, she faces a dreadful discovery: Chuck's mummified body and his severed head, preserved in a formaldehyde-filled jar, in a grotesque shrine created by Cheryl. As Cheryl's neighbor, Margie, grows suspicious and comes to investigate, she becomes another victim, savagely murdered by Cheryl with a machete. In the chaos, Sergeant Cook, who has been monitoring Cheryl's house, enters looking for Julia and meets the same dark fate after uncovering her in the basement.
Desperate and scared, Julia flees from Cheryl, leading to an intense confrontation that culminates in a pond where Cheryl again knocks her unconscious. Meanwhile, Billy awakens in the attic, now decorated with his childhood toys. In an attempt to seek help, he stumbles downstairs to call the police. However, Cheryl attacks him again, leading to a tense struggle where Billy ultimately impales her with a fireplace poker in self-defense.
In the aftermath, Billy reaches out to Tom for assistance. Detective Carlson arrives at the scene to find Tom treating Billy's injuries, only to discover Cheryl’s lifeless body sprawled on the floor. Enraged, Carlson turns his weapon on Billy and Tom, despite Julia's protests that Cheryl was the true culprit. In the ensuing struggle, Billy seizes the gun and, in a fit of panic, fires multiple rounds at Carlson, ultimately killing him.
Facing trial for Carlson's death, Billy is ultimately acquitted on the grounds of “temporary insanity.” The story closes as Billy and Julia embark on their college journey together, navigating the aftermath of their traumatic experiences side by side.
