Century of the Dragon.

Rookie policeman Shing (Louis Koo) grapples with right and wrong in this action-packed story about Hong Kong gangsters. Assigned to go undercover, Shing is so successful in playing the part of a gang member that he becomes right-hand man to gang leader Fei Ling (Andy Lau). Shing comes to respect Fei Ling and question his original motives, but the troubled cop must make a crucial decision before rival gangs and impatient cops move in for the kill.