A group of students are sent anonymous invitations to a 'ditch party' in the basement of their high school, only to find themselves trapped there when a rogue student, seeking revenge for years of torment, turns the entire campus into a free fire zone. As students and faculty members alike are mowed down with guns and homemade explosives, a school janitor caught in the crossfire, challenges the killer to a game of cat and mouse. But when the murderer manages to penetrate the fortified basement, the survivors confront an even more terrifying reality.