Kizuna Hida visits the strategic defense academy Ataraxia at his big sister's request. There, he encounters Aine Chidorigafuchi, a girl piloting the magical armor Zerosu. A communique about a serious mission arrives from Kizuna's sister. The mission is to rub Aina's chest. Kizuna has the power to level up a girl through indecent acts. With a battle against another world, the future depends on this power.