Chun Jae-in, an orphan-turned-police detective, is assigned to go undercover in a high school to befriend Seung-hee, the daughter of notorious gangster Cha Young-jae, and to shield her from anyone who might use her as bait to pressure her father while gathering information about his operations. At first, she is reluctant, but her uncle, a fellow detective, pushes her to take the assignment. Upon entering the class, she’s seen as a loser by the students, and a gang of girls squares off with her in the schoolyard. Chun Jae-in easily handles the fight, earning a hard-won measure of respect, though her real aim is to forge a genuine friendship with Seung-hee. That bond begins to grow with the help of Kang No-young, a handsome classmate and next‑door neighbor who becomes a crush—one that Jae-in questions because of their age difference.
A new teacher arrives at the school, and Jae-in learns, with growing unease, that the man is also an undercover agent working on her side. As she pieces together clues, she becomes suspicious of him, and some moments have her vaguely recognizing his face.
After school one day, the police spot Cha Young-jae outside an airport. They surge to reach him, but his rivals have the same idea. In a tense parking-lot confrontation, Cha Young-jae is boxed in, and a mysterious motorcycle rider cuts into the chaos, releasing white smoke that allows Cha Young-jae to slip away. The pursuing officers chase the rider, and the undercover teacher fires at him as well, but the driver escapes. Jae-in pursues, yet the rider eludes capture, and the undercover teacher presses a gun to her head. She senses a bluff and keeps going, only to notice a watch identical to Kang No-young’s on the driver. When she goes home and confronts No-young about the watch, he shrugs it off, and later we glimpse a bullet wound on his chest, hinting at a deeper undercover web.
Suspicion hardens into certainty for Jae-in, and she challenges Kang No-young to a Judo match during a school PE class. Between quips and whispers, the two reveal their true positions as undercover operatives. Jae-in finally confesses to Seung-hee that she isn’t really a schoolgirl, a revelation that deeply unsettles Seung-hee.
Tragedy strikes when her beloved uncle is stabbed by an unknown assailant. Grief-stricken, Jae-in is devastated as she and Seung-hee ride with a policeman toward the scene where Cha Young-jae is cornered. The officer betrays them, guiding them to the rendezvous where Cha Young-jae is meeting a rival gang leader. No-young arrives just in time, and Jae-in teams with him to fend off the attackers. She moves to arrest the rival leader, only to have an undercover teacher press a gun to her head and reveal that he was the one who stabbed her uncle. Enraged, Jae-in fights back as No-young, wounded and drained, cannot reveal his own identity without jeopardizing everything. A police officer finally cuffs the undercover teacher, and Cha Young-jae is rushed to the ER with Seung-hee by his side. Miraculously, Jae-in learns that her uncle’s condition is improving.
The story then shifts forward to another undercover assignment for Jae-in, though this time she adopts a dramatically different guise: a singing nun. She pursues a criminal through back alleys and streets until she discovers the suspect has already been knocked out by an unseen ally. Turning around, she finds No-young waiting, and the tension between them dissolves into familiarity as they share a kiss, signaling that their complicated mission—and their feelings—are far from over.
