Based on the erotic game by UnderMoon.

Masaya is a university student, who works as a home tutor part-time. One day, he receives a parcel. That is a notebook. He frowns when he reads a letter, which is also in the parcel. It says, "If you write a girl's name in the notebook, you'll be able to control her as you like." He thinks it is ridiculous at first, but he decides to give it a try. The name he writes is Yumi, his student.... As a result, it comes true, and he shags with her. "This notebook is proved to be real. I can now have fun with it...."

(Source: ErogeShop)