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Detective Choi In (played by Kim Sun-a) is a sharp and assertive female police officer. Due to her mission to investigate a gang, she is forced to return to school and pretend to be a high school student. Having been away from school for many years, her knowledge is naturally outdated compared to her peers, and she constantly tries to hide her shortcomings to avoid being exposed. A classmate, Huieung (played by Hong Soo-ye), also dislikes Choi In and makes her life difficult. Constrained by her undercover role, Choi In has no choice but to endure the harassment. However, she finds some happiness in her predicament when she develops feelings for a classmate named Noo Young (played by Kong Yoo). But when she considers her true identity and age, she feels dejected. The investigation finally begins to make progress, and the target starts to trust Choi In, allowing her to obtain crucial clues. However, the operation takes an unexpected turn.