Yonomori Kobeni is a high school student who leads a pretty normal life... and then the day of her 16th birthday arrives. Despite having very little presence, her fiance, Mitsumine Hakuya shows up suddenly along with her sister-in-law, a little girl named Mitsumine Mashiro. Without any prior explanation, they begin their peculiar life together. Kobeni's older sister, Yonomori Benio, who has has a sister complex and is a bit of a pervert is thrown into the mix and things get even weirder. Kobeni's life is no longer normal.