Midgar, all-girl academy, would have been notable just for the action of accepting its first and only male student, Yuu Mononobe. But Midgar stands out for much more than that: it's a school exclusive to a group of girls known as D's. Each of them have extremely powerful abilities in generating dark matter and manipulating it into powerful weaponry. The D's didn't exist twenty-five years ago, and only appeared after a number of mysterious, destructive monsters known as "Dragons" started appearing around the world. Strangely, just as suddenly as they appeared, they vanished. In their destructive wake, some girls started being born with symbols on their bodies and powers similar in nature to those wielded by the Dragons themselves. Now the D's attend this school, hoping to harness and utilize their powers against the Dragons. Yuu is their latest member and is extraordinary for being the only known male D in existence. Now he must forge relationships with the girls around him, including his long separated sister who attends the school as well, and work with them to investigate and eliminate the threat of the powerful Dragons.With the sudden appearance of the monsters that came to be called Dragons, the world had completely changed. Before long, within the human race supernatural girls with the power of the dragons, called "D," were born. The secret existence of the only male "D," the boy Mononobe Yuu, was forcefully thrown into the school where the girls of "D" gathered—Midgar, where he saw the naked body of one of the students, Iris. Furthermore, meeting his long-lost sister Mitsuki, Yuu seems to have been enrolled into this school...?! "When there's really no other choice, Iris, I will—kill you." "Can I really... believe you?" The curtains open as the "one and only story" about the boy who was supposed to become the strongest assassin and the incompetent girl unfolds—! Unlimited School Battle Action!