Nishi High is set to close in just two years, leaving only seven students enrolled. Hazakura Motoya, one of the last students, decides to stay and enjoy the free ride to graduation. Everything changes when three new transfer students — Kayama, Mizumura, and Toujou — arrive. To everyone’s surprise, these girls are members of a special military division. Soon after their arrival, a mysterious giant tower suddenly appears on the school grounds. Motoya is chosen by destiny to team up with the three girls and enter the tower. Together, they must uncover its secrets and face whatever dangers lie within.