The main character, a prefect, lives with his younger sister, Haruka. He loves Oppai and cosplay girls. Though he keeps it secret at school, there are a lot of cosplay uniforms in his room. Of course, Haruka dislikes him.... One day, she finds a tissue paper her older brother used for masturbation in a dust box. She feels sick at first, but she gets sexually excited when she smells it.... The main character sees it all and knows that Haruka always gets sexually excited by smelling semen....