Chan Wing Yan, a young police officer, has been sent undercover as a mole in the local mafia. Lau Kin Ming, a young mafia member, infiltrates the police force. Years later, their older counterparts, Chen Wing Yan and Inspector Lau Kin Ming, respectively, race against time to expose the mole within their midst.

A young couple engage in a suicide pact, and slit their wrists together. However, only the woman, who is already suffering from cancer, dies. Three donors give her lover a life-saving blood transfusion. Each of the three has a rare blood type, and each has his, or her, own secrets. The trio become a target for the girl's vengeful spirit. Before matters are set right, the three victims will have been forced to donate far more than their blood.

Champion competitive marksman Ken comes across an armored van robbery. He sees a policeman held hostage and shoots and kills four of the robbers. One of the robbers escapes and the policeman survives. The case is handled by Jerry Chang, whom Ken knows from having recently beaten him in a shooting match. Ken is found not guilty in court. Soon after, Ken is attacked by the escaped robber Pang Tao. Their confrontation reveals a very different background story and brings about a myriad of lies and traps and changes in relationships as Jerry and Ken try to outsmart each other.

In this prequel to the original, a bloody power struggle among the Triads coincides with the 1997 handover of Hong Kong, setting up the events of the first film.

Spoof of the Hong Kong Blockbuster Infernal Affairs, the film is based on a bizarre and nonsense war between the police and the triads. Lek Sir sends Keung, Yan, and Fatty undercover inside Sam's garage to get evidence of any crime Sam has committed, so Lek can win back the heart of Sam's wife Mary.