It was a very ordinary day at Morimiya Middle School when one of the students, a dark-haired schoolgirl with a gun and an ammunition belt, staged a shooting.

Ten minutes have passed since the first shot was fired.

A girl in a sports uniform and with a frightened expression on her face was running, waving a knife, down the hall, quickly closing the distance between her and girl shooter.

"My Senpai! I won't let him get hurt!"