Chun Jae-in, an orphan-turned-police detective, is assigned a job to go undercover in a high school to befriend Seung-hee, the daughter of notorious gangster Cha Young-jae, and protect her from any of her father's enemies who may want to use her as bait to get Cha Young-jae to do what they want, as well as get information about her father's work from her.

At first, Jae-in is very reluctant to accept the job; however, her uncle, also a detective, pushes her at the last minute to take it.

Jae-in is considered a loser when she first steps into her class, and the gang girls confront her afterward and challenge her to a fight at the backyard of the school.

She eventually gets this with the help of Kang No-young, a handsome classmate and next-door neighbor who she starts to like; however, Jae-in thinks her crush is wrong because of their age difference.

One day after school, Jae-in and other policemen spot Cha Young-jae outside an airport; they rush to catch him, but his rivals are also on the chase.

No-young shrugs it off and is playful; however, once he gets inside his apartment, he takes off his shirt, revealing a bullet wound to the camera.

The policeman betrays Jae-in and Seung-hee and leads them to the site where Cha Young-jae has been captured and is meeting with a rival gang leader.

Jae-in goes to capture the rival gang leader; however, as she is handcuffing him, a gun is put to her head.

Cha Young-jae is sent off to an emergency room in an ambulance with Seung-hee, and Jae-in receives news that her uncle will get better.