Koiro is a quiet, plain adult stepsister who normally stays expressionless at home in glasses and no makeup. One morning she appears with a touch of makeup, and a series of small changes brings her and her adult stepbrother closer. He finally confesses his long-hidden feelings; she objects because they are family, but agrees to thigh sex if he promises to stop there. Neither can forget the excitement, and rubbing between her thighs eventually becomes raw penetration and her first creampie. Once they cross that line, they cannot stop having sex and repeatedly join both body and heart.