Infernal Affairs 2 (2003): In this prequel to Wu jian dao (2002), Chen Ying Yan has just become an undercover cop in the triads while Lau Kin Ming joins the police force. Both the triads and the police find an enemy in a rival crime boss.

Infernal Affairs 3 (2003): In 1991. Yeung Kam Wing (Leon Lai) and Chan Wing Yan (Tony Leung) are both police school trainers. Chan was better than Yeung but because of violation of the rules, Chan was being expelled and Yeung graduated with the first in class. Yeung was employed and work in a different department from Lau's (Andy Lau). 12 years later, Yeung met his Chinese inspector buddy, Shen Chen (chen Dao Ming), both of them work under Police, sent to Sam (Tsang Chi Wai). Chan Jun, Yeung's spy, failed and wish to kill Yeung, but Yeung shot him. Lau arranged the car park seat number B3A6 to Yeung. Yeung asked Chan(tony Leung) who he is, but Chan didn't know Yeung. Yeung always met Sam and talk about police secrets, Lau recorded Yeung's conversation with Sam, and call the police head to Yeung's department, so that Lau can see how disgraceful Yeung is when the secrets are found. Shen Chen had changed Lau's tape back into Lau's cassette player, so when Lau play, it was Lau himself talking with Sam. Lau said that he wish to be a good police, wish to be a good person, why the police people always deceive him? Yeung, like Chan, spoke that Sorry he is police. Lau replied that he is also police and shot Yeung until Yeung fell down dead. Yeung said to Chan that in 1991 if not Chan got expelled, Yeung would never got the first in class. Lau met his girl (Sammi) and she got his baby. The psychologist met Shen Chen and talked.

Infernal Affairs (2004): Chan Wing Yan (Tony Leung), a hard-nosed cop, is sent undercover to infiltrate the notorious Triad crime ring. An expert at bringing down crime syndicates, Chan believes this next mission to be routine. What he’s not prepared for is the discovery that the Traid gang has a mole within his own police department – who is hunting Chan. What ensues is a battle of wills and only one cop can win.