In 1957, during a high school prom, Billy Nordham discovers his date, Mary Lou Maloney, in a compromising position with another student, Buddy Cooper. This betrayal leads to a bitter breakup, as Mary Lou admits she was merely using him for popularity. In a fit of rage, Billy decides to disrupt the ceremony by throwing a stink bomb onto the stage. However, his prank goes horribly wrong as the bomb ignites Mary Lou’s dress, leading to her tragic death in front of a shocked audience. In her final moments, she locks eyes with her assailant, Billy.
Fast forward 30 years, we meet Vicki Carpenter, who searches for a dress in the school prop room after her overly strict mother denies her a new one. In her quest, Vicki stumbles upon a trunk filled with the cape and crown meant for Mary Lou, unknowingly unleashing her vengeful spirit as she leaves the items behind overnight. That fateful night, Vicki's friend Jess becomes the first victim of the resurrected Mary Lou when she attempts to take the jewels from the crown; tragically, her death is misconstrued as a suicide attributed to an unwanted pregnancy.
As Vicki confronts increasingly surreal and disturbing visions involving Mary Lou, she reaches out to Buddy, who is now a priest. The situation escalates when Buddy's bible unexpectedly ignites after he tries to bless Mary Lou's grave, sending him into a panic. He turns to Billy, who has become the high school principal for help, but Billy dismisses Buddy’s claims of Mary Lou's return.
Vicki finds herself in detention after unintentionally slapping her rival, Kelly Henenlotter, during another of her hallucinations. In this detention, Vicki unwittingly becomes possessed by Mary Lou as she literally pulls the spirit through a blackboard that transforms into a whirlpool. Later, she confronts Buddy at church and, under the influence of Mary Lou, kills him before anyone can step in to help.
The next day, Vicki’s friends begin to notice her drastic change in behavior and fashion. One of them, Monica, confronts her in the locker room, only to meet a swift demise. Vicki embraces her dark transformation, taunting Billy about her possession, while he discovers Mary Lou's corpse is mysteriously missing from her grave, replaced by Buddy's.
At the prom, desperate for the title of prom queen, Kelly seduces Josh, who has control over the voting results. When he manipulates the votes to make Kelly the winner, Vicki, now fully aligned with Mary Lou's spirit, retaliates by electrocution through his computer, reversing the outcome. As Vicki is crowned prom queen, Billy, in a fit of desperation, shoots her multiple times in front of the horrified crowd. Immediately after, the fully resurrected Mary Lou bursts forth from Vicki, unleashing chaos and claiming Kelly as yet another victim.
As Mary Lou’s wrath escalates, she sets her sights on Craig, Billy's son, chasing him into the prop room with a malevolent intent to drag him to the underworld using the trunk that once contained her spirit. In a surprising turn, Billy intervenes with Mary Lou’s crown, seemingly pacifying her horrific spree. Once free of possession, Vicki emerges from the trunk, reuniting with Craig, while Billy offers to drive them home, assuring their safety. However, in a chilling twist, once they are all in the car, Billy reveals that he, too, is now possessed by Mary Lou, leaving Vicki and Craig in sheer terror as he drives off into the night.
