During the summer holidays, a student named Katsuragi, at the request of his late grandfather, got a job with a young lady Tsubareta as a butler. There he met Tsubareta the Cat. She was charming, and he couldn't help but pay attention to her.

Hentai is based on the manga by Osuzu Akiomi. The first part tells about the forbidden love between a brother named Anya and his younger sister Kasumi.

One evening, Seiichi went out onto the balcony to find inspiration for his new romantic work. He decided to get some air and look at the night city. Suddenly, he noticed that something strange was happening on the next balcony behind a thin wall. There was Maki, his classmate, whom he considered a sweet, kind and intelligent girl. She was doing something that was completely unacceptable to him.