In Infernal Affairs II, the story is set ten years before the events of the first movie and focuses on the early careers of the main characters Chan Wing Yan (played by Edison Chen) and Lau Kin Ming (played by Shawn Yue).
Chan Wing Yan is a member of a triad gang, but he's also secretly working as an undercover cop for the Hong Kong police. He's full of ambition and determination, but he quickly finds himself struggling to balance his loyalty to the police with his obligations to the gang.
Meanwhile, Lau Kin Ming is a young police officer with a troubled family history. His father was also a cop, but he died in the line of duty, leaving Lau to take care of his disabled mother. When Lau joins the police force, he's determined to prove himself and make his father proud.
As the two men work their way up the ladder in their respective organizations, they begin to cross paths more and more frequently. They both get involved in a complex web of corruption and betrayal that puts their lives and careers in danger.
The main villain of the movie is a ruthless crime boss named Sam (played by Eric Tsang). Sam is a major antagonist in both Infernal Affairs II and the first movie, and his actions drive much of the plot. He's constantly trying to undermine the police and eliminate any threats to his power, including both Chan and Lau.
Throughout the movie, we see how Chan and Lau's characters evolve and how their actions will eventually shape the events of the first movie. The film is filled with suspenseful action sequences, emotional character development, and plenty of twists and turns.