Cai Yin (Kim Sun-ah) is a smart and aggressive policewoman. Because of the task of tracking down the gang, she has to return to school and be a high school student. After being away from school for many years, Cai Yin's knowledge is certainly not up to par with others, and she protects herself everywhere for fear of being seen through. Hui Ling (Hong Xiu'er), a big sister in the class, also dislikes Cai Yin and embarrasses her everywhere. Due to her undercover identity, Cai Yin had to swallow her anger. What makes Cai Yin happy from the obstruction is that she has a good impression of a classmate named Lu Rong (Kong Yu), but thinking of her true identity and age, Cai Yin feels frustrated. The action has finally made progress, and the target of the investigation began to believe Cai Yin, and she has obtained important clues, but the action has changed again.