Ryo Mizuhara and his sister Akane Mizuhara were living peacefully without their parents (since their parents died a few years ago). But suddenly a girl named Shizuka Masuda barged into their house and claims to be Ryo's fiancée. She explains to him that it's because "Japanese government's secret childbirth program". She transfers to his school and is in the same class as him, although she is one year younger.