Leung Siu-Tong is an experienced undercover agent who returns from a long-term assignment in Mainland China to teach new undercover recruits at Hong Kong Police's Criminal Intelligence Bureau (CIB). During this time, he befriends triad member Michael So, who only agrees to the relationship because he sees it as a stepping stone to the role of gang leader. Meanwhile, conflict at the CIB department ensues when Siu-Tong forms a tentative romantic relationship with his supervisor Jodie Chow.