The taciturn and unassuming sister-in-law, Koiro. At home, she's always expressionless, without makeup, and wearing glasses. One morning, she appeared with a bit of makeup on. The brother who cares for his sister and the small changes she makes in response. The brother can't hold back any longer and confesses that he has always liked her. A sister who refuses because they are siblings, but the two promise to only do dry humping and rub their crotches together. Although it ends once, the excitement from that time cannot be forgotten between the two, and eventually, they end up having intercourse. Once they start having sex, they can't stop and repeatedly engage in physical and emotional intimacy.
