Harbour City, somewhere in Asia. Southern Precinct Homicide Division detective Hero Wu (Mark Chao) catches a gang of security-van robbers and makes headlines with his flashy, maverick methods. As a result, he is sent for anger-management counselling and temporarily taken off any cases. Meanwhile, Hsu Ta-fu (Huang Bo), a member of Sanlien triad who's looking to retire, decides to make some extra cash for himself while his boss is away in Vietnam for five days. After borrowing US$1 million from Sanlien colleague Yuan (Jack Kao), he plans to cut a deal on some illegal diamonds, sell them immediately for US$1 million profit, and quietly return the original capital to Sanlien untouched. Unfortunately, a paramilitary SIS squad arrives at the handover of the diamonds to Ta-fu, and in the shootout both the case with the diamonds and the one with Ta-fu's US$1 million go missing. Hero, who is following events against his boss' orders, arrests Ta-fu and the two go in search of the missing items. Also on their tail is SIS Captain Ou (Alex To), who wants both Hero and the case with the diamonds. Staying one step ahead of Ou thanks to help from Hero's colleague Midori at the precinct, Hero and Ta-fu follow a trail that leads from Fan Ning (Angelababy) to Ta-fu's object of desire, Tu Hsiao-ching (Terri Kwan), to mysterious broker Jabar (Leon Dai). En route they discover the secret concealed by the diamonds and a complicated conspiracy involving the Pandawa terrorist organisation, Harbour City's organised crime and top government officials.