Mitsuhiko attends Seiyou Gakuen and one night, his childhood friend Rio was going to confess to him when there was a loud noise. He sees his classmate Koiboshi on the rooftop with a blank look on her face. Not listening to their calls, she falls from the roof. He runs to catch her, but her body is surrounded by a mysterious light and he falls unconscious. When he wakes up and found himself and the school in a very deep hole. He gathers the remaining students and teachers and begins to find a way out of this hole.