City of Damnation (2009)

Jang Choon-dong is a traffic officer who aspires to become a homicide detective. He is assigned to go undercover to infiltrate a gangster network. Meanwhile, Lee Joong-dae, a member of Kwang-seob's gang, goes undercover himself by joining the police force. Joong-dae tries hard to gain favor with his squad leader in an effort to expose Choon-dong's identity.