1960's Hong Kong was plagued by widespread police corruption, with gambling dens and middlemen forming a powerful network of illicit profits within the police force. Over ten years, bribes to the police reached staggering amounts of billions in Hong Kong dollars. The street-smart undercover officer Johnny Long (played by Chow Yun-Fat) employed ruthless methods to crack cases and exert pressure from above, resulting in the brutal beating of university student Han Jiang Bong (played by Michael Hui). His mother sought help from the corrupt police officer Porky (played by Wong Jok-Hwa), who managed to rescue Han.
The flamboyant and arrogant Chief Inspector Kuochen Lei (played by Liam Neeson) was feared for his ruthless methods. Despite being kind-hearted and perceptive, Chen Feiniu (played by Andy Lau) rose from a common beat cop to the head of the Fanling District within five years, but at a great cost - his family secretly harbored nine women connected to the triad members...
In 1974, Hong Kong's corruption scandal shook the international community and led to the establishment of the ICAC. Long-time police officer Chiang Han (played by Tony Leung Ka Fai) and disillusioned undercover officer Kuochen Lei were among the first batch of ICAC agents, appointed to bring an end to Hong Kong's underground rules.
In the midst of this struggle against corruption, Chen Feiniu, who was on the brink of bankruptcy due to a bribery scandal, unexpectedly found his one true love...
