One fateful evening at Herrington High School in Ohio, after a discussion regarding the school’s budget, several teachers and the principal, Valerie Drake, leave for the night. However, after realizing she left her keys behind, Valerie returns only to be trapped in her office by the increasingly unstable Coach Willis. In a shocking turn of events, he stabs her hand with a pencil. As she struggles to escape, she discovers that the entrance door has been chained shut, encountering the drama teacher Mrs. Olsen outside. Despite retrieving her keys and a pair of scissors, Valerie cannot evade her fate, as Mrs. Olsen callously stabs her to death.
The following morning, we're introduced to six central characters. Casey Connor is portrayed as a diligent, albeit clumsy, geek and photographer for the school newspaper. Meanwhile, the cunning Delilah Profitt, the paper's editor-in-chief and head cheerleader, exudes an air of superiority. Her beleaguered boyfriend, Stan Rosado, contemplates leaving the football team for academics, while Zeke Tyler, a smart yet rebellious student stuck in his senior year, dabbles in illegal activities, including selling a homemade ecstasy-like drug. His actions concern the timid Miss Elizabeth Burke who worries for his future.
A naive transfer student, Marybeth Louise Hutchinson, aims to befriend self-proclaimed outcast Stokely Mitchell, despite Delilah spreading rumors about Stokely being a lesbian. Surprisingly, Marybeth develops a crush on Zeke. While having lunch, Casey stumbles upon a peculiar creature in the grass and brings it to science teacher Mr. Furlong, who suspects it might be a new species of cephalopod-specific parasite.
Delilah and Casey hide in the teacher's lounge, where they witness a horrifying scene: Coach Willis and Mrs. Olsen attempting to assault school nurse Ms. Harper by forcing a creature into her ear. They also discover the lifeless body of Mrs. Brummel. After a frantic escape, Casey alerts authorities, but Principal Drake dismisses his concerns as a cry for attention.
As paranoia takes hold, Casey theorizes that the teachers may be under alien control. When Zeke and Marybeth enter, their skepticism is met with aggression from Mr. Furlong. A struggle ensues, leading to Zeke making the drastic decision to cut off the teacher's fingers before using his drug on him.
Zeke later takes the group back to his house to investigate the bizarre creature, leading to the discovery that it requires water to survive and can be killed by his drugs. To test the theory, Zeke forces everyone to take his drug to ensure they are not infected, but Delilah reveals her true nature by destroying Zeke's lab and escaping.
The next night, while the school football team is infected, they decide to take down Principal Drake, believing her to be the alien queen. After a fatal confrontation, Stan falls victim to the infection as well. As Zeke and Casey search for drugs in Zeke's car, they are ambushed by more infected students.
Back at the gym, Stokely grows suspicious of Marybeth, who ultimately reveals herself as the alien queen. After a harrowing chase, Zeke is thrown against lockers, but Casey cleverly lures Marybeth into a trap. In a last-ditch effort, he stabs her with the drug, resulting in her demise. Although Casey is briefly infected, he quickly reverts to normal as the alien queen disintegrates into dust.
A month later, life at Herrington High returns to normal. Stan and Stokely start dating, while Zeke takes Stan's place on the football team, much to the delight of Miss Burke. Miraculously, Mr. Furlong survives, albeit with an eyepatch, and Casey finds romance with Delilah Profitt as he gains local hero status.
