In 1991, Liu Jianming (Andy Lau), a member of the Hong Kong triad gang, followed the instructions of the boss Han Chen (Eric Tsang) and joined the police force to become a gang undercover. Han Chen promised that Liu Jianming would help him to be promoted to a trainee inspector in seven years. In 1992, Chen Yongren (Tony Leung), an outstanding student of the Police Training School, was asked by his superiors to go deep into the triad as an undercover, with the ultimate goal of becoming a celebrity around Han Chen. In 2002, both of them lived up to their expectations and were under great pressure. Liu Jianming gradually wanted to become a real good person, while Chen Yongren was looking forward to returning to the police identity as soon as possible. The serious case team learned a batch of drug trading intelligence from Chen Yongren, locking the target figure Han Chen, but the intelligence was leaked by Liu Jianming, and both actions failed. However, this incident exposed the fact that both sides had undercovers, triggering the determination of both sides' high-level officials to eliminate the ghosts. Liu Jianming and Chen Yongren, who had different and similar destinies, began to face severe tests in their journey of infernal affairs.