The story happened in a county of a group of top students in an experimental class. Twelve years ago, an experimental class mysteriously disappeared. From then on, there were no classes on the fifth floor of the first teaching building, and the teacher never talked about it. But twelve years later, the sister of a girl in the class who disappeared that year also entered that high school. In the first semester of her sophomore year, the sister also disappeared mysteriously. A month later, she was said to have been murdered. Two boys in her class accidentally went to the fifth floor for some reason, but were involved. What happened twelve years ago gradually became clear...