She first crossed the line with her adult son's friend Yu five years ago. That night, after discovering her husband's third affair and having a bitter fight, Yu comforted her and innocently asked whether she would let him help her get revenge. Though guilty about sleeping with a younger adult who was playing a student role at the time, she could not reject his feelings. His warmth revived desires she had forgotten, and they continued secretly seeking each other out. But graduation and his departure eventually bring a major change to their relationship.