First-years Nagi Souichiro and Bob Makihara entered Tendou High School with the intents of taking it over- only for Souichiro to get soundly defeated by Natsume Maya, the sexy captain of the Jyukenbo martial arts club, and who can change her body at will to that of her child-self. Nagi Souichiro is futher humiliated in his complete loss against Maya's subordinate, Takayanagi Masataka. Things get more complicated as Natsume Maya's little sister, Natsume Aya, asks the girl-shy Souichiro to marry her, and tells him to promise never to lose another fight...

A day or so after their violent debut at Tendou High School, which turns out to be a martial arts centered institution, the Enforcers, the school discipline team, moves to punish the upstart freshmen.

It doesn't take long before Souichiro and Bob realize their weakness in comparison to Natsume, Takayanagi, and the Enforcers, and join the Jyukenbo with the promise of becoming stronger than any of the current members, within three months.

Natsume Aya, in conflict with the Enforcers, who's president is Takayanagi Masataka's older brother, welcomes them in hopes of Nagi Souichiro finally realizing her goal...