The younger sister deeply loves her older brother and adores him more than anything. One day, she catches her best friend having sex with her brother in private. Overwhelmed by jealousy, she confronts her brother and passionately demands that he do the same with her. Though initially hesitant and wanting to refuse, the brother is moved by his sister's intense passion and can no longer resist, giving in to intercourse. This single impulsive moment becomes the turning point that drastically changes their relationship forever.