Eri lives an ideal family life, but for some reason, her boyfriends always treat her coldly due to her "strong sex drive." Seeking comfort from the emotional pain, she turns to her younger brother, Ma-kun, who treats her with kindness. One day, she finds herself captivated by Ma-kun's strong, beautiful body and is overwhelmed by her uncontrollable desires. Calling it "sex education," she begins treating her own brother as an outlet for her urges. This forbidden impulse becomes her only source of healing, marking the beginning of a new, taboo relationship.