She’s on Duty

Chun Jae-in is assigned to go undercover in a high school to befriend Seung-hee and protect her from rivals who might use her as bait. Her uncle, a fellow detective, pushes her to take the job, and she reluctantly agrees. The setup establishes the central mission and the personal risk she faces.

Jae-in enters the class and is immediately targeted by the gang girls. They confront her in the school's backyard and challenge her to a fight. She defeats them with ease, earning respect while still seeking Seung-hee's friendship.

Jae-in meets Kang No-young, a charming classmate and next-door neighbor, and begins to develop feelings for him. His presence complicates her undercover mission as she navigates a growing attraction with an age difference. They gradually connect despite the double life she leads.

A new teacher arrives at the school, and Jae-in learns he is also an undercover agent on her side. She grows suspicious and vaguely recognizes his face, adding tension as potential double agents populate the staff. The discovery foreshadows betrayals to come.

After school, police spot Cha Young-jae outside an airport and race to intercept him as rivals close in. A mysterious motorcycle rider speeds into the scene, releasing white smoke to allow Cha Young-jae to escape. The undercover teacher opens fire, but the rider gets away, increasing the danger.

Back home, Jae-in confronts No-young about a watch that looks identical to his, hinting at a double life. He shrugs off the suspicion, but later she discovers a bullet wound on his body when he returns to his apartment, deepening her fears that he is undercover as well.

During a physical education class, Jae-in and No-young duel in a tense judo match. Their verbal sparring gives way to a realization that they are both undercover agents. The moment shifts from suspicion to wary trust as they acknowledge each other’s true roles.

Jae-in confesses to Seung-hee that she is not really a schoolgirl, a revelation that upsets Seung-hee and strains their friendship. The admission sharpens the personal stakes of the case and tests Seung-hee’s trust in Jae-in.

Jae-in learns her beloved uncle has been stabbed by an unknown attacker, leaving her in tears as she mourns the loss of her only family. The emotional blow raises the stakes for the undercover mission and personal safety.

In the Cha Young-jae operation, a policeman betrays Jae-in and Seung-hee, guiding them toward danger. No-young arrives to help, and Jae-in fights to secure Cha Young-jae as rivals close in on the leader. The moment escalates as the undercover teacher makes his move.

The undercover teacher reveals he is the betrayer and is the one who stabbed Jae-in’s uncle, turning the confrontation against her. No-young is wounded and must remain quiet to avoid exposure while he regroups. A police officer finally restrains the teacher, ending the immediate threat.

Cha Young-jae is rushed to the emergency room and Seung-hee is saved. Jae-in learns that her uncle will recover, lifting the immediate sense of loss and signaling a fragile but hopeful ending to the investigation. The characters regroup to reflect on the cost of their double lives.

Some time later, Jae-in is again undercover as a singing nun, pursuing a criminal through alleys. She discovers No-young has knocked out the target, and they reconcile with a kiss, choosing to face the future together despite their secret lives.