Kengo is an ordinary high school student. One day, a new student, Mikage, transfers to his class. She is unfriendly to everyone so that she can't blend in this class. Triggered by an accident, Kengo makes friends with Mikage. A student in his class finds out students in other classes are disappearing one by one. Kengo and Mikage patrol around the school but they notice they can't get out of their school building. Students are trapped in the school building and they are getting....