a scared Robin accidentally falling to her death through a second-story window. The children make a pact not to tell anyone what happened and keep the incident a secret and they leave. Just then, the shadow of an unseen person who witnessed Robin's death crosses over her body. Six years later in 1980, Robin's family attend her memorial on the anniversary of her death. Robin's teenage sister, Kim, and fraternal twin brother, Alex, are preparing for the school prom to be held that evening. Kelly, Jude, and Wendy begin receiving anonymous obscene phone calls, while Nick ignores his ringing phone. Kim and Nick are now dating and plan on attending prom together. Jude is asked by goofy jokester Seymour "Slick" Crane, who she meets by chance. Kelly is going with Drew, her boyfriend. Wendy—Nick's ex-girlfriend—asks school bully Lou to the prom with the sole purpose of embarrassing Nick and her rival Kim. In the changing room after gym class, Kim and Kelly discover the locker room mirror cracked with a shard missing. Later, Wendy, Jude and Kelly each find their yearbook photos stabbed with a piece of glass. Meanwhile, Kim and Alex's father (also the school principal) learns that the sex offender blamed for Robin's death has escaped from a psychiatric facility. Lt. McBride, Nick's father, investigates his disappearance. During the senior prom, Kelly and Drew make out in the changing room, but Kelly, a virgin, refuses to have sex and Drew angrily leaves. As Kelly gets dressed, an unidentified figure wearing a ski mask and all-black clothing approaches her and slits her throat with a mirror shard. Jude and Slick have sex and smoke marijuana in his van parked outside school grounds. They are attacked by the masked killer, who stabs Jude in the throat. Slick struggles with the killer, who jumps from the vehicle before Slick drives off a cliff to his death. Staking out the prom, McBride is informed that the sex offender blamed for Robin's death has been caught. Now wielding an ax, the killer confronts and chases Wendy through the school. Evading the killer several times, she screams when she discovers Kelly's body in a storage room and is hacked to death. The alcoholic school janitor, Sykes, witnesses Wendy's murder and attempts to notify the school staff, but they dismiss it as a drunken rant. Meanwhile, Kim and Nick prepare to be crowned prom king