Idealistic police cadet Chan Wing-Yan (Tony Leung) is recruited by Police Superintendant Wong (Anthony Wong) to go deep undercover as a member of the criminal Triad society. Into the same cadet class, crime boss Hon Sam (Eric Tsang) installs new Triad member Lau Kin Ming (Andy Lau) to become a long-term mole for the gang. A decade later, both have risen through the ranks in their respective aliases -- but when both of their covers are in danger of being blown, a game of life and death ensues.