The ideal owner of beautiful breasts, a tsundere little sister. I want to obtain everything and make her deredere. The protagonist is the 'Student Council Disciplinary Committee Chair' whom even the teachers respect, but he has a side that is a breast fetishist and loves cosplay (making others do it). Haruka dislikes her perverted older brother who is completely different from how he is at school, and outside of school she often talks back or even hits him. Then one day Haruka discovers tissues used for masturbation in the trash. Out of curiosity she ends up smelling them and, without even realizing it herself, gets a little aroused. The protagonist sees that and begins taking action to turn her into an obedient, erotically cute little sister.