Nymphomania

The protagonist, Hayama Keisuke, is a most average student at a private school. And although they are not related by blood, he has an excellent relationship with his stepmother and sister.
Owing to his timid nature, he has not yet dated any girls, but he has an adequate - no, considerable - interest in sex for a boy of his age.

Indecent thoughts of his childhood friend - his classmate Yumi - never leave his mind.

But then, a single girl turns Keisuke's mundane life on its head.

Although change is yet only creeping up on him, it has nevertheless begun. And when Keisuke realizes what is about to happen, his dull life becomes increasingly disturbed day by day.

There is no way back.