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]