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.

In the prequel of Infernal Affairs. Chan Wing Yan has been expelled from police academy in cause of his relatives to the triad. Now SP Wong give him a chance to undercover the triad family controlled by his half brother Hau. Besides of Ming. He has been ordered to killed Hau father and infiltrated the police department. The story get complicated when Wong s related to Hau father s dead. The avenge is begin when Mary. Sam s wife is the hit order. Now everything is complicated and related

Because Chan was being expelled from the school in 1991, SP Yeung Kam Wing graduated, with all police reports distinctions, being the first in whole class. Twelve year later, he was being sent, working to Lau. Ten months after Chan dies, Yeung s spy had blew a case and the spy wish to kill Yeung, but Yeung took his gun and shot him to conclude this failure. Lau gave Yeung car park seat B3A6 when Yeung met Lau. Yeung asked Chan who he is, but Chan didn t remember him. Sam works with Yeung and Lau is recording Yeung s words so that he can sue Yeung. Yeung is a police SP, which is being sent to Sam as the spy from police. Lau doesn t really know Yeung s Police SP position and wish to catch Yeung to the police head. Yeung work with another inspector who is a good friend of Yeung himself. Lau sneaked into Yeung s office, stealing the cassette, and ask the head to tell him that Yeung is the spy of the triad...