Prequel to the Hong Kong gangster thriller 'Infernal Affairs' (2002), set ten years before the events of the original film. Chen Ying Yan (Shawn Yue) has just become an undercover cop in the triads, while Lau Kin Ming (Edison Chen) is a new recruit to the police force. Bitter enemies from the first film, Inspector Wong (Anthony Wong Chau-Sang) and Sam (Eric Tsang) are behaving like old friends, while Yan is embroiled in the family saga of a triad cartel, and Ming has developed an oedipal fixation on his boss's wife. As friendships fracture and foes join forces, nothing is as it first appears to be...