Infernal Affairs (2003):
Synopsis :- Triad Kingpin Sam (Eric Tsang) has ruled the Hong Kong Underworld for a good five years with an iron fist. To keep his dominance on the crime world, Sam comes up with the most brilliant plot - send some Triad spies to become police officers. Among others, Ming (Andy Lau), a fresh 18-year-old recruit, is sent in as a mole in the Hong Kong police department. Fearing that Sam's power is growing by the day, Inspector Wong (Anthony Wong) sends his top cadet, Yan (Tony Leung) as an undercover cop to bring down Sam's Triad family.
Ten years later, Yan's ruthlessness as a gangster brings him to the top as Sam's right-hand man. Ming's deviousness, and with Sam's help, also rises as Hong Kong's top cop. But these two guys soon play a deadly game of cat and mouse by trying to snuff the other out. Things are complicated when Ming, after being a cop for so long, wants to do good and Yan, after chopping one too many thugs, gets confused which side he's on . . .

Infernal Affairs II (2003):
Synopsis :-
After retired Triad boss, "Uncle Kwan" is murdered in cold blood, the five remaining underbosses are geared up to make the first move should the Kwan family collapse amidst the chaos. The police is on high alert as a bloodbath between the Kwan family and the underbosses struggle for power. But who did Uncle Kwan name as his successor? Of the five bosses, who would rise to fight for the throne? To everyone's surprise, Uncle Kwan's son Hau (Francis Ng), rises to the challenge. Well educated and soft spoken, Hau proves to be a ruthless adversary as he dismantles the underbosses singlehandedly and proves to be the kingpin his father only wished he could become. As the police aim for one target, the silent Hau has become their worst nightmare. Their only hope left is to target Hau's baby brother . . . Yan (Shawn Yu).

Infernal Affairs III (2004):
synopsis :-
Ten months have passed since the murder of Yan (Tony Leung) and Ming (Andy Lau) is now relegated to the admin section of the police department for secretarial duties while the investigation of the shooting continues. Of course, with Ming's previous history of flawless credits and a host of awards as a policeman, he gets away scott free and moves back into the office as head of Internal Affairs. But when several new cases involving high profile corrupt cops land on his desk, Ming immediately takes note. He is ordered to investigate the head of the Security Dept., Inspector Yeung (Leon Lai). Extremely ruthless, Yeung has risen well above Ming's ranks as a cop and also has ties to Sam, Yan, and a Godfather from China named Shen (Chen Daoming) who is known for drug trafficking and arms smuggling. But lately Yeung is linked to several "suicides" and other cop killings - all of which are later proved to be Sam's moles. Could Yeung be "the mole" of moles that Sam referred to previously? The man inside that Sam depends on more than Ming himself?