Our protagonist, Umino Kousuke, is a third-year student at Shunpu Academy. His father is on a business trip overseas, and his mother has been staying with his father since April, so he currently lives with his step-sister, Kotomi. After three months, he's gotten used to living alone with her, but his mother soon will return.

Kotomi is petite, 155cm tall, but she has a pretty big bust...no, huge breasts. What's more, she seems to be madly in love with the protagonist.

The "accident" he had hoped for didn't happen, but that doesn't mean Kousuke dislikes Kotomi. But he's the older brother. Kotomi is the younger sister. He's a contrary character who can't be honest with himself.

In an attempt to win over her "big brother," Kotomi climbs on top of him every morning to wake him up, and when they go to school, she crosses his arms and presses her chest against him, aggressively attacking him. So, what will be Kousuke's fate...?

[From Getchu]