In 1991, Hong Kong gang leader Ni Kun was killed. Except for Han Chen (Eric Tsang), all the five big bosses under the Ni family were waiting for an opportunity to betray and attack. Unexpectedly, they were easily restrained by the inconspicuous second generation of the Ni family, Ni Yongxiao (Francis Ng). This move aroused the vigilance of the chief inspector Huang Zhicheng (Anthony Wong), who sent police academy student Chen Yongren (Shawn Yue) to infiltrate the Ni family for a long-term undercover work. In order to consolidate his power, Han Chen had already planted an informant Liu Jianming (Edison Chen) in the police. Three years later, Ni Yongxiao, whose career is booming, decided to annihilate the five big bosses in one fell swoop to rest assured, but due to negligence, Han Chen slipped through the net. In 1997, Han Chen, who survived a disaster, returned to the underworld. In order to avenge his dead colleague, Huang Zhicheng approached him to discuss cooperation in getting rid of Ni Yongxiao. However, he soon discovered that this move was a big mistake. Chen Yongren and Liu Jianming, who were each troubled by their embarrassing identities, went further and further on the road of Infernal Affairs.