Adult role-play. Mizuki and he are not blood relatives; they met as adult student step-siblings after their parents remarried. Years later, after both parents died in an accident, they lived together and supported each other. As their life alone continued, he fell in love with Mizuki and dreamed of remaining by her side. That dream is threatened by his best friend Kyohei, who wants Mizuki for himself and approaches her whenever her stepbrother is away.