Julie receives several telephone calls from a heavy “Breather” as she babysits at the Hummers’ home, and is startled when her boyfriend, Charlie, sneaks into the house. They go upstairs to have sex, but Julie insists he take a shower first. Meanwhile, downstairs, the heavy Breather, wearing green gloves and black boots, rummages through an assortment of murderous instruments on the Hummers’ desk and chooses a paper clip, then stabs Julie to death. Charlie returns to the bedroom, and screams as a large green garbage bag attacks him. Later, the Hummers discover Julie’s body in their bed and Charlie’s corpse stuffed in the garbage bag. At the graveside funeral for Julie and Charlie, Principal Peters acknowledges the other events scheduled for the day, including classes, a parade, a football game, and the prom. Malvert, the janitor, steps forward wearing green gloves and black boots, and Peters warns him not to look so suspicious. Nearby, Toby confides to her friend, Hardy, that she warned Julie not to let Charlie help babysit. They watch as Joe and Bertha sneak away from the service to have sex in Joe’s car. Realizing he forgot to bring protection, Joe leaves to purchase condoms. As Bertha waits in the car, the killer attacks her with a wooden horse head bookend. Joe returns with his condoms and is murdered by a stranger with a garbage bag. Later, at Lamab High School, as Mr. Dumpkin, the wood shop teacher, instructs his class on how to make horse head bookends, Toby shows Hardy the bloody horse head bookend she found at the crime scene. Dumpkin seizes the bookend and compliments Toby on her wood staining skills. Later, in the girl’s locker room, the heavy Breather observes the girls changing clothes. When the others leave, Toby hears the Breather and runs away, but is frightened when she meets the strange-looking Malvert, who asks for assistance on a crossword puzzle. When the parade begins, Toby, Hardy, Principal Peters, his assistant, Miss Mumsley, and several teachers are in the crowd. A student, Ralph, rides with the float featuring fellow student, Dagmar, atop the school’s mascot, a bull. Ralph persuades Dagmar to join him inside the float for sex. Dagmar complains the vehicle is moving too fast, so Ralph hurries to tell the driver to slow down. While he is gone, the killer climbs inside and kills Dagmar with an eggplant. When Ralph returns, he is attacked by a garbage bag. Worried about her friends, Toby climbs atop the float and drops inside as the bodies are tossed out the back. Seconds later, when Toby crawls out, onlookers accuse her of murder. In the principal’s office, Toby is interrogated by Peters, the sheriff, Nurse Krud, and several teachers. Peters sends Toby for a psychiatric evaluation by Dr. Sigmund. As Toby leaves, Malvert urinates in a corner trash can, and Peters explains that Malvert was a teacher but sustained serious injuries in an accident and became the janitor. The heavy Breather telephones, disguises his voice by speaking through a rubber chicken, and announces he will kill someone at the football game. Meanwhile, Dr. Sigmund questions Toby about sex and her home life. She denies killing anyone and demands to return to class. As she enters her English class, Peters announces over the intercom that Dr. Sigmund thinks Toby might be the killer. Outside the school, Miss Mumsley asks Malvert to confess to the murders if suspicion falls upon Principal Peters. At the football game, Nurse Krud informs Dumpkin that the boys can die in a garbage bag without being killed first. To prove her point, they tie a student, Mawamba, in a bag. However, they are distracted by the football game, forget to free Mawamba, and he dies. Meanwhile, students, faculty and Malvert watch the game, and everyone notices when Joan and Al sneak under the bleachers to have sex. Joan is disgusted by the trash falling from the stands, so Al searches for a blanket and a broom. Despite Hardy’s protestations, Toby goes to check on her friends, slips beneath the bleachers, and is knocked unconscious by falling trash while the killer murders Joan with a chalkboard eraser. Al returns, and is murdered with a garbage bag. When Toby regains consciousness, she overhears plans to find and arrest her. As the corpses are loaded into a van, Toby sneaks inside, and Hardy follows the vehicle on his bike. A short time later, Hardy is questioned by Peters, the sheriff, and several teachers. Attempting to shock Hardy, the sheriff lifts the sheet covering Al’s body on the gurney. The others avert their eyes, so only Hardy sees that Toby also lies under the covers. The killer telephones again to reveal he will attack at the prom. After the adults rush out, Toby insists on going to the prom to clear her name, and stops at the drama department to disguise herself in a sexy outfit. Toby believes the clues are locked in Peters’ office, and she needs the principal’s keys, but cannot get them off his belt. Patti suspects Toby is hiding beneath the disguise, so Toby kisses Hardy, and Patti is fooled, believing the virginal Toby is incapable of such passion. Their kiss excites several teachers, including Dumpkin, who leaves the prom and heads to the wood shop. While Hardy sneaks to investigate Miss Mumsley’s office, Toby approaches Malvert, and persuades him to get Peters’ keys for her. Meanwhile, Miss Mumsley announces that since all the prom queen candidates, except Patti, are dead, the teachers voted to award the crown to Principal Peters. Patti is upset, so her boyfriend Scott suggests they have sex, and Dumpkin hides in the closet as the couple sneaks into the wood shop. Scott realizes he does not have a condom and leaves to get one. Patti is murdered with the prom crown and when Scott returns, he is killed with a garbage bag. Dumpkin’s closet is attacked with a chainsaw. Dumpkin recognizes the murderer, tries to convince him not to use the chainsaw, and is killed with the chainsaw’s case. Toby sneaks into Peters’ office and accidentally unlocks a drawer of marbles, which spill onto the floor. As she finds evidence, Peters enters and admits he killed the female students, who were “naughty” for having sex. Despite Toby’s claim that she is a virgin, Peters attempts to kill her with a school trophy, but slips on the marbles and impales himself. Toby runs into Miss Mumsley’s office and discovers Hardy’s body. Miss Mumsley, claiming to be Peters’ father, admits to killing the male students to dispatch any witnesses to her son’s murders. Toby runs away through the halls strewn with garbage bags, and is horrified to find her dead friends stalking her. Panicking, she falls out a window and drops several stories, but is awakened by Hardy’s kiss, and finds herself in a hospital bed, recovering from swine flu. Her friends are still alive and the adults are very different people than in her dream-state, especially Mr. Mumsley. Later, as Toby and Hardy stroll through the woods, she reveals Dr. Sigmund’s diagnosis that swine flu is caused by sexual repression, then suggests they have sex. While Toby undresses, Hardy slips on green gloves and strangles her. One hour later, at Toby’s funeral, Hardy kneels to place flowers at her grave, but Toby’s arms reach up through the dirt and grab him by the throat.