In this prequel to Mou gaan dou (2002), Chan Wing Yan has just become an undercover cop in the triads while Lau Kin Ming joins the police force. Both the triads and the police find an enemy in a rival crime boss.

Infernal Affairs III is a 2003 Hong Kong crime thriller film directed by Andrew Lau and Alan Mak. It is the third installment in the Infernal Affairs film series, and is both a sequel and a semi-prequel to the original film, as it intercuts events before and after the events in the original.