Shinichi visits his grandparents on the countryside. There he meets a sickly ojousama, Asami, and got reacquainted with a girl he doesn't recall so well, Mari.

What he thought to be a calm romantic summer vacation turns out to be a shameful erotic mishap when a mischievous girl pulled a prank on him. He's tricked to eat a mushroom that causes him to have erection round the clock.

Shinichi the erection prince, as townsfolk calls him, swore vengeance. But first of all, he has to calm his penis down.