Working as an undercover for many years, Laughing ( Michael Tse ) escapes from all calamity. Two years ago, he underwent special training to become a mute and worked in mainland China as an undercover. Closely working with the mainland Chinese police, Laughing successfully busts an illegal armed weaponry organisation and resumes his identity as a police officer in Hong Kong. He is appointed as head instructor of the Criminal Intelligence Bureau (CIB), specialising in seeking and training CIB recruits in undercover work.

Laughing, however, is unable to adapt to the stress in the discipline of police life and becomes a rogue cop. To get his jobs done quicker, he hires the gangster Michael So ( Bosco Wong ) as his informant . On the surface, Laughing and Michael appear to be good brothers, but in fact both have their own purpose, mutually taking advantage of one another. Feeling as if Laughing is holding him back from "moving up" in the triad, Michael betrays Laughing and decides to work alone. To tackle down the triad's crimes, Laughing applies to be an undercover once again.