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The three ''Infernal Affairs'' films are incredibly twisty [[TheTriadsAndTheTongs cops vs. Triads]] films set in HongKong between 1991 and 2003. The first film, set in 2002, is based around the stories of two [[NotSoDifferent mirrored]] infiltrators. Chen Wing Yan is an undercover cop in the gang of the chubby, AffablyEvil gangster Hon Sam. Yan's handler is [[TheChessmaster Chessmaster]] senior cop Wong, who doesn't realise that his most trusted underling, Lau Kin Ming, is actually a mole planted in the police force by Sam years before. As Wong's attempts to bring Sam down reach their endgame, Yan and Ming both find their covers in danger of being blown and are both ordered by their pretended bosses to catch the "moles" who are actually themselves.

The second film is a {{Prequel}} that tells the story of how Sam rose to his position in the first film, and the unexpected roles that Yan, Ming, and Wong played in that. The third film is a sequel that follows the stories of the surviving characters from the first film, and also tells a flashback story set just before it. It introduces two new significant characters to replace the dead people: Yeung, a smooth and sinister, possibly corrupt, cop from the Security branch; and Shen, a mysterious gangster from the Chinese mainland.

Elements of all three films were streamlined into the [[ForeignRemake remake]] as ''Film/TheDeparted'', which transferred the action to Boston and made the Triad equivalents [[TheIrishMob Irish-American gangsters]] (with a bit of Irish Mob mythology about [[http://en.wikipedia.org/wiki/Whitey_Bulger Whitey Bulger]] tossed in), and won the 2006 Best Picture Oscar. In Japan, an episode aired in [=TBS=] in 2012 as a remake under the title ''Double Face - Sennyuu Sosa Hen''.

Oh, and the title and the epigrams at the beginnings and ends of the films suggest that HongKong is Buddhist {{Hell}} and all the characters' lives suck because they're being punished for the sins of their past lives. Or they metaphorically evoke the hells that Yan and Lau are living in as a result of their undercover rules.
!!The film series provides examples of:

%%* AffablyEvil: Sam, mostly.
* AndIMustScream: In the ending of the third film, [[spoiler: Ming in a wheelchair paralyzed and in a catatonic state.]]
%%* BecomingTheMask
* BeingEvilSucks: If you're a gangster, you will probably end up an utterly corrupt, friendless wreck of a human tormented by guilt over your crimes, and you'll probably die young and unpleasantly.
* BeingGoodSucks: If you're a cop, you know that society is essentially corrupt, that any gangster you manage to put away will get rapidly replaced, and that you'll probably die young and unpleasantly.
* BigBad: The first film has Hon Sam, the Triad boss ordering the infiltration.
* BilingualBonus: Dr. Lee's name is a pun on psychologist in Cantonese.
%%* BlackAndGrayMorality: Especially in the second and third films.
* CharacterTics: Both Yan's tic of tapping his fingers on things, and Ming's of tapping objects he's carrying against his leg as he walks, are plot points.
%%* ChekhovsGun: [[spoiler: The envelope]] in the first film.
%%* TheChessmaster: loads, but especially Wong, Sam, Sam's wife Mary, Ming and the younger Ngai.
%%* CloudCuckooLander: Keung has shades of this.
%%* CoolShades
%%* CowboyCop
%%* DeathIsDramatic
%%* DestinationDefenestration: [[spoiler: Wong's death.]]
%%* DetectiveMole: Ming.
%%* DirtyCop
* DrivenToSuicide: In third film, [[spoiler: after Ming inadvertently revealed himself as Sam's mole, he snapped and killed Yeung before gets wounded by Shen. Ming then attempts suicide by shooting himself through his lower jaw, too bad the bullet doesn't fatally wound him.]]
* EverythingsLouderWithBagpipes: Played at cop funerals.
* GambitPileup: All three films, but especially the plotting in the second film involving Wong, Sam, Sam's wife Mary, and Ngai Wing-Hau.
* GenreShift: The first two films are gangster movies, the third is a MindScrew psychological horror.
%%* GoingNative
* HereWeGoAgain: the third film ends with a flashback to the period of the first film that finishes with Yan and Ming unknowingly meeting each other in a hi-fi store - the scene that began the main action of the first film after an establishing pre-credit sequence.
%%* HeroicBSOD: Yan, [[spoiler: after witnessing Wong fall on the taxi he just got off.]]
%%* HiddenWire
%%* HiredToHuntYourself: Both Yan and Ming in the first film.
%%* IKnowYouKnowIKnow
* IfICantHaveYou: [[spoiler: Lau sold the first Mary out to the Ngais when she refused to return his romantic attentions.]]
%%* InternalAffairs
%%* KansasCityShuffle
* KillItWithFire: How one of the capo in second film got disposed of.
%%* LadyMacbeth: Sam's wife Mary.
* LikesOlderWomen: [[spoiler: A younger Lau towards the first Mary, Sam's wife.]]
%%* LookBothWays: [[spoiler: The death of Sam's wife Mary]]
%%* MexicanStandoff
%%* MindScrew
%%* MobWar
* TheMole: The two leads are the most obvious example. [[spoiler: Shen is an undercover cop, and Billy acts as a mole for the triads.]]
* NapoleonDelusion: [[spoiler: In the third film, Ming turns out to have gone insane and to believe that he is now Yan.]]
* NoMrBondIExpectYouToDine: In a morally-inverted example, Wong likes doing this to Sam whenever he's arrested.
%%* NotSoDifferent
* OhCrap: Wong, when Del Piero manages to halt the elevator he's in.
* OneSteveLimit: Exception with the two Marys.
%%* OutGambitted
%%* OutWithABang: The capo "Gandhi" in the second film.
* TheCuckooLanderWasRight: All through the three films, Keung has always been the one to point out undercover cops. A few times, he has also made throwaway comments pointing to Yan as a cop.
* ThePurge: The massacre of the Ngai capos in the second film, [[spoiler: and Ngai's family near the end of the movie.]]
%%* ThoseTwoGuys: Del Piero and Keung in the first movie.
%%* ReverseMole
* RevisedEnding: The mainland China version of the first film ended with a different ending in which [[spoiler: Ming gets caught]], because of government MoralGuardians.
* RooftopConfrontation: Yan and Ming after they learn of each other's identity.
%%* RoomFullOfCrazy
%%* TheShrink: Yan's therapist Dr. Lee.
%%* ShootTheHostageTaker
%%* SinisterShades
%%* SunglassesAtNight: Shen (and all the other sunglasses tropes on this page)
* TheScapegoat: Billy. Ming pins everything on him at the beginning of the third film.
%%* ThroughTheEyesOfMadness: [[spoiler: Most of the third film.]]
* {{Unperson}}: [[spoiler: Ming erases Yan's police file after Yan discovered Ming's double identity as Sam's mole.]]
%%* XanatosSpeedChess