An ordinary family of four: The father, Tomoyuki, an ordinary office worker. The mother, Rinne, a full time housewife. The elder brother, Taketo, and the younger sister, Maasa, separated by a few years. Siblings that usually fight, before he realizes he has become aware that his sister's body has begun to change. And so, one summer day: "Hey.... do you want to try it?" Taketo says teasingly licking his sister's body. "Ohhh... ahh... onii-chan...!" Unexpectedly, his sister has a pained reaction. But Taketo continues his relentless passionate assault, and so little by little his younger sister's body begins to crave the experience... [Translated from the official website]