Ah Kun (played by Andy Lau) has long monopolized the heroin market. Although he regards Ah Li (played by Daniel Wu), who has been following him for more than 8 years, as his successor, Ah Kun has always been cautious and has never revealed the location of the "warehouse" or the processing plant, nor has he met other "colleagues". Ah Kun intends to hand over his business to Ah Li, and Ah Li gradually learns more secrets in the drug market. For 8 years, Ah Li has been collecting all the information of drug dealers as an undercover agent, but he has fallen in love with the drug addict Ah Fen (played by Zhang Jingchu). Ah Fen's situation makes him feel more confused about his identity as a marginal person. In the process of fighting and fighting, Ah Li finally gained Ah Kun's complete trust, and the 8-year pursuit also came to an end.