When I returned home for the first time in a while, I heard that my step-sister, who was created by my parents' remarriage, was apparently a shut-in, so I visited her out of concern. It seems that Koiro was teased about having big breasts, and that's why she's a shut-in... When I try to do something brotherly and comfort Koiro, her expression gradually changes... "We're step-siblings, so it's okay," and I, who am a bad person, succumb to her charm and am seduced by her big breasts.