Naoki Shinoda is the main character of Hentai. He does all the housework and cooks for his single mother Arisa Shinoda. Naoki’s parents got divorced when he was still a child. His father cheated on Arisa, and she did not forgive him. Naoki’s mother bought a new apartment for alimony, which was located next door to her childhood friend Shiori Nishimura. They lost contact with each other during their school life. But now they are close again and share snotty memories with each other. One day Arisa decides to go on a big trip, Naoki sets the table and they celebrate her departure together with Shiori. During the feast, Arisa asks her best friend to take care of her favorite son. Soon Shiori finds Naoki while masturbating while watching porn. She has not had sex for almost ten years. So she decided to train Naoki for a while and at the same time to quench her.