From Baka-Tsuki:

"It is a boring club", Sasaki Itto was interested in entering the Movie Club just because of such a light and lazily written club introduction. He was 2 months late starting school because of his family's problems, and with hope and anxiety he entered the club's room. Inside the room, there is Hasegawa Shiho who is dancing while holding a toy gun, and the arrogant Kudou Ozuma. "Get the hell out of here! You have nothing to do in this club" - was the greeting Itto got from Ozuma.

But on the way home from school, Itto gets attacked by a guy with a huge weapon. By coincidence Shiho was also there and the attacker changes his target to attack Shiho. In the urge to save her from the attacks, Itto's power as a Parabellum awakened...