On the eve of the gift-giving holiday White Day, new Yeondu High School student, Lee Hee-min, sneaks into school to hide a box of candies for the girl of his dreams. But when the shutters slam shut, he finds himself trapped in the building overnight, hunted by killer janitors and haunted by the troubled souls of the dead. Can Hee-min remain hidden while searching for clues to uncover the dark secrets of the building's past? Or will the horrors that stalk the school's eerie hallways by night claim his life before he can escape?