Laura Ramstead, a senior athlete at a small-town Southern California high school, collapses during a track meet and dies from a cardiac embolism. Two months later, Anne Ramstead, her elder sister and a U.S. Navy officer, arrives in town to participate in the graduation ceremony honoring Laura. Anne blames Coach Michaels for pushing Laura too hard, which she believes contributed to the tragedy. As she wanders through downtown, she sees Paula, Laura's teammate, head toward the wooded jogging trails before Paula is found murdered by an unseen killer.
Anne attends a graduation rehearsal at the high school, where she is a special guest. Afterward, she visits Laura's boyfriend, Kevin, and hands him one of Laura's medals as a memento. Meanwhile, Sally, a gymnast preparing for a photoshoot, is startled in the locker room by her classmates Doris and Joanne, and she cannot perform her routine. In the same locker room, a figure dressed in fencing gear corners Sally and drives a fencing sword through her throat.
The following day, Ralph comes across Doris and Joanne on the trails in the woods. They tease him by taking his football and tossing it into the bushes, but when he searches for it, the killer returns the ball with a sharp metal spike, impaling him. That night, a graduation dance unfolds at the high school. Dolores, a flirtatious track teammate, leaves the dance with her boyfriend Tony and the couple retreats to the woods to be alone. They are confronted by the killer, who decapitates them both with a sword.
Principal Guglione receives calls from worried parents—Paula, Sally, Tony, and Dolores have vanished. The inspector, Insp. Halliday, arrives to investigate. Later, on the track field, Pete attempts a pole vault, only to crash onto pads that have been swapped for steel spikes and die. Halliday questions Coach Michaels about the disappearances; Michaels, enraged, reveals he has just been fired because of the negative publicity surrounding Laura’s death.
Doris and Joanne eventually discover Sally’s body stuffed in a locker in the locker room. Their screams draw Coach Michaels, who finds the scene with Kevin. Kevin accuses Michaels of being the killer, and a physical struggle ensues. Anne and Halliday arrive and notice a photo in the locker showing every team member’s face crossed out—except Kevin’s. Michaels flees into the wooded trails, pursued by Kevin, who reveals himself as the killer and confesses that he has been targeting the track team to blame them for Laura’s death. A tense fight ends with Halliday shooting Michaels to death, assuming him to be the culprit.
Later, Anne goes to Kevin’s house. Upstairs she discovers Laura’s corpse in Kevin’s bedroom, dressed in a graduation cap and gown. A deranged Kevin charges at Anne, intent on killing her so she won’t “take Laura away.” During their struggle, Anne hurls Kevin—and Laura’s corpse—through the window. She escapes to the high school track field, where Kevin pursues her under the bleachers and discovers Dolores’s severed head and Pete’s spike-riddled body in a storage shed. In the confrontation, Anne manages to shove Kevin onto Pete’s spike-littered corpse, killing him.
After giving her statement to the police, Anne returns home, visibly shaken, and hallucinates an undead Kevin in her room. In reality, it is her drunken stepfather, Ronald Corliss. The next morning, Anne quietly leaves town in a taxi as the Graduation Day banner looms over the main street, signaling the end of a deadly chapter for the students she once cheered on.
