The nerdy Park Young-Joon is performing at a talent show during a high school field trip to Gyeongju, where he is booed off stage and replaced by the class brawler Choi Gi-Dong. A massive fight ensues with a local gang, and Gi-Dong is admired because he leads his class in the rumble while the timid Young-Joon is ostracized because he was the only student who didn't participate.
Several years later, Gi-Dong and Ju-Sup meet each other. Young-Joon has become a top gangster while Gi-Dong, who is now a teacher, tries to prevent Ju-Sup and other students from joining the mob. The two former schoolmates bond over drinks and both go to the police station when Ju-Sup and his friends are arrested for fighting.
At the station, they meet Ju-Sup's beautiful big sister, Ju-Sup. Their friendship is strained as they compete for her affections; they also differ over Ju-Sup, since Gi-Dong wants to keep him away from Young-Joon, while the gangster wants to encourage the student to study.
While the competition between the two rivals becomes increasingly fierce, the police are investigating Young-Joon and other mobsters who are plotting against him. Gi-Dong and Young-Joon join forces when someone kidnaps Ju-San, and eventually Gi-Dong's friends, the mob, the police, and the student body get involved.