The assassination of Triad kingpin Kwun in 1991 unleashes a power struggle among other Triad bosses. While they are all geared up to make the first move should the Kwun's family collapse into chaos, Sam (Eric Tsang) remains the only loyalist. Meanwhile, the police under the command of Inspectors Wong (Anthony Wong) and SP Luk (Hu Jun) have been placed on the highest alert. A bloodbath is bounded to take place.
To everybody's surprise, the ultimate winner in this scramble is a most unlikely candidate: Kwun's son Hau. An accountant in training, Hau has concealed his cunning ruthlessness under an almost self-effacing facade. Almost effortlessly, he manages to restrain all the other Triad bosses to become the new kingpin.
To watch over Hau's every step, Wong sends rookie Yan (Shawn Yue; adult character played by Tony Leung in Infernal Affairs) as a mole to infiltrate into his organization. It turns out that Yan is actually Hau's half-brother who has severed all ties with his father. Inside Hau's organization, Yan befriends Keung (Chapman To), one of Sam's underlings. Meanwhile, in order to strengthen his position, Sam has sent a young subordinate Ming (Edison Chen; adult character played by Andy Lau in Infernal Affairs) to enroll in the police cadet academy.
Three years have gone by. Yan has become a trusted member in Hau's organization while Ming has risen up to top ranks in the police force thanks to information supplied by Sam. In the meantime, Hau has become more ambitious than ever. He sees to it that all the other Triad bosses are taken down at the same time. Even Sam is targeted for elimination in Thailand.
Hau has also found out that in order to help Sam climb up the echelon in the Triad world, Sam's girlfriend Mary (Carina Lau) has colluded with Inspector Wong years ago to assassinate his father. When ming learns that Mary, on whom he has a secret crush since time immemorial, is in physical danger, he tries his best to rescue her, only to see her killed with his naked eyes. With Mary killed, Wong becomes the next target. When his best friend Luk dies in his stead, Wong decided to fight back on pain of death.
In 1997, Sam suddenly makes a surprise comeback in Hong Kong. It turns out that he has survived Hau's assassination attempt and has been hiding with Keung and others in Thailand. Wong has tracked him down and has convinced him to come back as a witness against Hau...