In the small town of Wallingham, three teenagers, Tom, Freddy, and Michelle, break into the dilapidated high school to collect an artifact with the school's mascot, a warthog. In the field, Tom finds the mascot's costume, and puts it on, only to transform into the warthog, which then kills Freddy, and Michelle. Meanwhile, the ambitious Sabrina Swanson is planning her ten-year high school reunion in the school with her two friends, and former classmates Margo, and Ronnie. As the party begins, she clashes with her high school arch-rival Rosario Stewart. Outside, the warthog kills two guests, shorting out the electricity in the process.Sabrina goes to check the power box, but sees the warthog, and warns Ronnie, but continues with the party. Meanwhile, Margo explores the building with her high school boyfriend Jud, but they are attacked by the warthog; Ronnie saves Margo, but Jud is killed. They warn Sabrina of the danger, and Rosario soon finds out as well before the four witnesses the warthog killing one of the guests. As they attempt to evacuate the building, the warthog invades the party, and kills nearly everyone. Sabrina, Rosario, and party-goers Meg, and George find refuge in an equipment room while Margo, and Ronnie save elderly teacher Mrs. Blakeslee.Mrs. Blakeslee reveals to Margo, and Ronnie that the warthog has previously killed people before being killed herself. Margo, and Ronnie escape to the library, and begin researching the school's history before eventually discovering that the son of the school's founder went on a killing spree in costume when he was kicked off the football team; anybody who has put on the costume since has transformed into the warthog. Sabrina, Rosario, Meg, and George arm themselves with various weapons, but Meg, and George are killed in the escape as Sabrina, and Rosario go back into the building to rescue Margo, and Ronnie.Sabrina, and Rosario reunite with Margo, and Ronnie, who tell them what they've found. The warthog soon attacks them, and kills Margo, while Ronnie is killed fending it off so Sabrina, and Rosario can escape. The girls make it to the field, where they set up a trap for the warthog. Working together, they eventually manage to kill it. As Sabrina, and Rosario walk off the school grounds, Tom's decapitated body is seen lying on the ground.