As a child, Brooke Storm goes to play with her dolls in a shack along a biking path in the marsh near her home. A man and his girlfriend arrive by motorcycle, and Brooke hides behind the shack while the man violently attempts to force his girlfriend to have sex with him. When she refuses, the two depart on his motorcycle. As they turn around and pass by the shack, Brooke throws a bucket of wet mud at them, causing him to lose control of his motorcycle, and both are killed when they are impaled on nearby fenceposts.
Eighteen years later, Brooke lives alone in her childhood home, and is haunted by the deaths she caused as a child, which are now considered unsolved murders. Brooke works as a domineering and neurotic high school biology teacher who is unliked by her students. One of them, senior Dickens, is particularly antagonistic toward Brooke, and when he taunts and humiliates her during an exam, she slaps him in the face. Dickens and his friend Jon-Jon follow Brooke home in their car, and spy through her bathroom window while she masturbates in the shower.
The next day, Jon-Jon goes to pick up his new waver classmate Queenie at her home to attend the football game, where they meet with Dickens and their prankster friend, Smiler. The four disrupt the football game by driving Dickens' car onto the field before fleeing. Dickens devises a nebulous revenge prank against Brooke, and brings his friends to the marsh where the young couple were killed eighteen years prior. There, they gather swamp mud and then approach Brooke's house, donning Halloween masks. They climb onto her roof and begin pounding their feet, and throwing mud at the windows. When Brooke steps outside, they dump a bucket of mud on her, triggering traumatic memories of the deaths she caused as a child.
The teenagers flee when Brooke's friend Mink arrives, finding Brooke in a paranoid fugue state. Mink provides Brooke with a quaalude to calm her nerves. After she leaves, Dickens enters the house and finds Brooke nearly incoherent. When Dickens and Queenie begin to sexually assault Brooke, Jon-Jon and Smiler intervene. A fight ensues, during which Brooke leaps from her bed and jumps through a second-story window. Assuming Brooke to be dead, Dickens sends Jon-Jon to the local diner to steal a belonging from one of the school's football players, which they can leave behind in Brooke's house, framing the football players for her murder. Jon-Jon slashes the quarterback's tires and steals a jersey from his car. The quarterback and his teammate chase after Jon-Jon on a motorcycle, but lose control and crash.
Meanwhile, Queenie angrily leaves the house. Outside, she finds Brooke crying in the woods. Queenie approaches Brooke, relieved to see that she is alive, only for Brooke to bludgeon her to death with a rock. When Jon-Jon goes to investigate, Brooke also bludgeons him, rendering him unconscious. Brooke returns inside the house, where she stabs Smiler through the temple with a pencil, killing him. When Dickens goes to help him, Brooke lunges down the staircase with a butcher knife, and stabs Dickens in the chest. Jon-Jon regains consciousness and returns inside the house, where he finds an injured Dickens bound to a wall, where Brooke is preparing to perform a live autopsy on him. Jon-Jon incapacitates Brooke and frees Dickens, but she soon awakens. Armed with the butcher knife, Dickens leaps on top of her, and is impaled with a firepoker she is wielding; Brooke's throat is slashed in the process. Jon-Jon leaves the quarterback's jersey in the house, and returns home and hides in his bedroom.
At school the following day, police arrive when Brooke fails to appear. They enter the biology class, overseen by a substitute teacher, and swiftly arrest the school quarterback. Jon-Jon is momentarily relieved, until he hallucinates that Brooke is the substitute teacher.
Brooke, driven by trauma and provocation, violently dispatches several of her tormentors, culminating in a bloody confrontation with the remaining students.
The film’s ending brings Brooke’s childhood trauma full circle, as the violence she witnessed and inadvertently caused as a child is mirrored in her adult actions. The cycle of victimhood and vengeance plays out in a grimly poetic fashion, turning the tables on the instigators and leaving the audience to ponder the cost of cruelty and repression.
