Koiro, the quiet and unassuming sister-in-law. At home, she wears glasses and has no makeup on, but one morning she shows up wearing just a little makeup. Her brother is concerned about her, and in response, small changes pile up. Unable to contain himself, the brother confesses that he has always loved her. The sister refuses, saying they are brother and sister, but the two end up rubbing their crotches together, promising to end it with just a bare thigh. Although it ends once, the excitement of that time is still rekindled between them...