小悪魔発育チュ～！

The protagonist, Nakashimizu Hiro, goes back to his home when summer vacation begins. He is a university student who is seen as serious but well-liked by people around him. As a consequence, he's been trusted to teach as a private tutor for his cousin Hikaru and his childhood friend Rina, who actually have secret feeling for him. This also applies to his friend’s little sister Manami who often come to his house. His little sister Sana also respects him very much. Yet, he never dates any of them, or even had a girlfriend for once. The reason is that he actually has a secret he takes to the grave: he likes getting approached by assertive, younger girls.

However, one day the girls find out about his fetish in an accident! And now all the girls who were previously meek become aggressive towards him, including his little sister!?