In 1959, Yau Ma Tei foreign detective Reeve retires, leading to a competition between bitter rivals Lee Rock (Andy Lau) and Ngan Tung (Paul Chun) for the position. Although Ngan ultimately gets the position, Lee is promoted to Chief Chinese Detective, a position that supersedes Ngan in seniority. After Lee takes office, he reforms each department, gaining support from many businessmen and his momentum rises while Ngan's standing plummets, fueling his hatred towards Lee. He hires a henchman from mobster King Crab (Michael Chan) in a failed assassination ploy against Lee. Fearing he will be discovered, Ngan has Crab killed. Lee continues to consolidate power and throughout the 1960s, gains complete control of the entire Hong Kong Police Force and its operations whilst actively investing in commercial real estate, entertainment and many other fields. He comes to amass an estimated wealth of HK$500 million.
In 1972, British officer Sutcliffe is appointed the new Commissioner of Police, though he quickly uncovers mass corruption within the Hong Kong Police and swears to oust all its criminal activities. Two years later, anti-corruption agency ICAC is established. Faced with the encroaching threats to his livelihood, Lee plans to transfer all of his assets to Canada and immigrate there with his wife Grace (Sharla Cheung) and their daughter. He vows to make Rose(Chingmy Yau) his second wife and during a reunion with her, learns she has had his son, Bill. However, a law against polygamy passes in Hong Kong and, hoping to avoid being the third party between Lee and Grace, Rose chooses to leave with Bill and settles in Australia.
Years pass and Rose and a now adult Bill (Aaron Kwok) return to Hong Kong where they reunite with Lee. Though Rose hopes Bill will accept Lee as his father, Bill, being a member of the ICAC and believing Lee abandoned his mother to fend for herself, vows to arrest him and put an end to his illicit operations. Broken-hearted from seeing the animosity between father and son, Rose collapses and is hospitalized. Lee rushes to the hospital but en route is injured, narrowly escaping a car-bombing planted by King Crab's younger brother, who desires to avenge his brother and had been lead to believe he had been killed by Lee by Ngan. Lee and Bill witness Rose's death at the hospital and Bill, out of respect for his mother, lets Lee leave. However, both are attacked by assailants and fend them off, culminating in a final standoff between Lee and Ngan. When the latter taunts Lee to arrest him, Bill intervenes on his father's behalf and apprehends Ngan.
The film's epilogue depicts various corrupt police officers being arrested by the ICAC while the wanted Lee enjoys a stable life with his family in Canada.