You've just started college, every time you spot a female figure, your gaze wanders and you start to feel your dick throbbing. In the faculty library you come across (or it finds you) a notebook: the Sex Note. The name promises and you don't hesitate to take it home.
When you open the notebook in your room, Yuuna appears, a sexy Yokugami (Goddess of Desire), owner of the Sex Note, who made you find the notebook in order to have fun.
When the Goddess of Desire appears, you are in for a shock. She explains who she is and that with the notebook you can fuck whoever you want. You are hopelessly turned on by both the Yokugami and the usefulness of the notebook.