Yui, who lost her mother, lives with her father and younger brother Ma-kun. One day, Ma-kun gets mocked by his friends for lacking sexual knowledge. Seeing this, Yui feels responsible to teach him the facts of life in place of their late mother, and decides to personally educate Ma-kun about sex. However, afterward, Ma-kun grows increasingly bold, causing the situation to spiral into deeper complexity. A shocking family drama unfolds, exploring the fragile bond between siblings and the turmoil of crossing intimate boundaries.