The three Infernal Affairs films are incredibly twisty cops vs. Triads films set in Hong Kong between 1991 and 2003. The first film, set in 2002, is based around the stories of two mirrored infiltrators. Chan Wing Yan (Tony Leung Chiu-wai) is an undercover cop in the gang of the chubby, Affably Evil gangster Hon Sam (Eric Tsang). Yan's handler is Chessmaster senior cop Wong Chi-shing (Anthony Wong), who doesn't realise that his most trusted underling, Lau Kin Ming (Andy Lau), 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 Kam-wing (Leon Lai), a smooth and sinister, possibly corrupt, cop from the Security branch; and "Shadow" Shen Cheng (Chen Daoming), a mysterious gangster from the Chinese mainland.
Elements of all three films were streamlined into the remake as The Departed, which transferred the action to Boston and made the Triad equivalents Irish-American gangsters (with a bit of Irish Mob mythology about Whitey Bulger
tossed in), and won the 2006 Best Picture Academy Award. In Japan, an episode aired in TBS in 2012 as a remake under the title Double Face - Sennyuu Sosa Hen. It was adapted in South Korea as City of Damnation and in India as Homam in the Telugu language.
There's a TV series sequel with the same name, which takes place years after the end of the third movie.
The film series provides examples of:
- Actually Pretty Funny: In the first film, Sam is eating dinner in the police conference room, and makes a joke about the food not being poisoned. Wong replies, "There's always dessert," which Sam laughs at.
- Affably Evil:
- Sam in his good mood. Even more so in the prequel, as he was Wong's best friend.
- In the prequel, Hau is introduced as a thoughtful, melancholic and educated man. But then, he kills a lot of people.
- And I Must Scream: In the ending of the third film, Ming is left paralyzed and catatonic.
- Bait-and-Switch Accusation: Del Piero and Keung tell Yan to his face that he is a cop. He looks surprised for a second. Then they reveal that it was only a joke.
- Batman Gambit: A lot of them throughout the films, one example being towards the end of the first film, where Yan and Lau plot Sam's demise. They make him think Keung was the mole. Since Keung knew where the hiding place for his drugs was, he is in great haste to get there and remove the drugs, taking Yan with him. With this, Yan can report the way to the hiding place to Lau, so the police can go after him and surround him. He is then shot by Lau.
- Becoming the Mask: Downplayed with Yan, as he was struggling to keep himself from becoming his own mask. Zig-zagged with Ming, who wants to embrace his identity as a cop, even if it means indirectly causing the death of Wong and Yan and the killings of Sam and Billy to cover his past.
- Beeping Computers: The police computers make sounds when searching for and finding stuff.
- Being Evil Sucks: 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.
- Being Good Sucks: 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.
- Big Bad: The first film has Hon Sam, the Triad boss ordering the infiltration. In the prequel, Hau Wing Ngai is the big bad even for Hon Sam himself.
- Bilingual Bonus: Dr. Lee's name is a pun on "psychologist" in Cantonese.
- When Wong was handing over a watch to Yan, he mentioned the pinhole camera was located at number 5, reflecting Yan's birthday and his status as an undercover. note
- While filling out their personal information, Keung asked Yan about the second letter of the word bodyguard in Chinese. Yan almost got the right letter, only to made a mistake afterwards. Doubles as a funny moment.
- Broken Pedestal: Wong fell out with Sam after the events of the second film, establishing their rivalry. Sam even foreshadows this in the beginning, while he was dining with Wong in police station.
- Car Cushion: A deadly one for SP Wong in the first film.
- Character Tics: 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.
- Chekhov's Gun: The envelope handed by Sam to Ming, Yan recognized it judging by the handwriting he left on it.
- The Chessmaster: Most of the main characters, most notably Wong, Sam, Sam's wife Mary, Ming, Yan, Hau, and Shen and Yeung as a duo.
- Compromising Call: In the first film, when Yan follows Lau out of the theatre, a phone call keeps him from learning the true identity of Han Sam's mole.
- Cool Shades: Shen and Wong.
- The Cuckoo Lander Was Right: 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.
- Dark and Troubled Past: Ming is constantly struggling with his double identity as Sam's mole while he attempts to embrace his identity as a police officer.
- Yan was an illegitimate son of Ngai Kwun, a triad boss and father of Ngai Wing-hau. Despite being estranged, Yan's tie to him almost got him expelled from the police academy for real until Wong offered Yan a second chance.
- Dead Man Honking: Upon his death, Keung sinks onto the steering wheel and the honking starts.
- Death Is Dramatic: Whenever a major character is killed off, a flashback sequence of the killed character is played along with a melancholic background music.
- Deceased Fall-Guy Gambit: When Keung dies, he is labeled by the police as an undercover cop in order to take the heat off Yan.
- Desk Sweep of Rage: When he gets angry, Hon Sam wipes the food off the table at the police station in the first film.
- Destination Defenestration: Superintendent Wong gets thrown off the rooftop by Sam's goons after he refused to compromise Yan's identity.
- Detective Mole: Ming was tasked by Wong to find out the mole within the police force, which also happens to be himself.
- Dirty Cop: Ming and Billy as they're both Sam's mole in police force. Yeung was suspected to be one of this due to his tactics and his connection with Shen, a mainland gangster. Turns out Shen is an undercover cop working for the mainland government and is cooperating with Yeung to expose Ming's real identity.
- Driven to Suicide: In the third film, 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 deliver the fatal blow.
- Everything's Louder with Bagpipes: Played at cop funerals.
- Evil Plan: Hon Sam seeks to gain information on the Hong Kong police by placing moles in their ranks.
- External Combustion: Superintendent Luk died to a car bomb set by Ngai, which was intended for Wong as a revenge for his role in his father's murder.
- Fingertip Drug Analysis: In the first film, Yan approves the imported cocain after a quick snort.
- Four Is Death: When Yan and SP Wong are waiting at the elevator, the digital floor counter skips the 4th floor.
- Gambit Pileup: All three films, but especially the plotting in the second film involving Wong, Sam, Sam's wife Mary, and Ngai Wing-Hau.
- Genre Shift: The first two films are gangster movies, the third is a Mind Screw psychological horror.
- Grey-and-Gray Morality: The main theme of the whole series, but especially evident in the prequel. On one hand you have a cop masterminding a mob boss' murder and on the other you have the son of the mob boss attempting to go legit.
- Heel–Face Turn: A lot of them by several characters throughout the trilogy. The most notable is in the first film: After Wong's death, Lau decides that he doesn't want to serve Sam any longer. He confederates with Yan to stalemate Sam and kill him, hoping he will be able to leave his past as a gangster mole behind and fully become a cop.
- Here We Go Again!: 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.
- Heroic BSoD: In the first film, Yan experienced one after witnessing Wong landed on the taxi he just got off.
- Hidden Wire: Ngai found this in his half-brother, Yan's jacket in his dying moment.
- Hired to Hunt Yourself: In the first film, both Yan and Ming are ordered by their apparent boss to find the mole in their team, while actually being that person.
- Human Shield: Yan used Lau as a cover to get out of the building. It doesn't work.
- If I Can't Have You…: Ming sold the first Mary out to the Ngais when she refused to return his romantic affections.
- Kill It with Fire: Man-cheng, one of Ngai's capo in the prequel was dealt with this way.
- Likes Older Women: A younger Ming towards the first Mary, Sam's wife.
- Look Both Ways: Sam's wife, Mary was ran over by Ngai's gangsters at the airport after Ming betrayed her.
- Mexican Standoff: In the perhaps most famous scene of the series, Yan has Lau at gunpoint while Billy threatens Yan likewise.
- There is also a similar situation at the end of the prequel where Hau threatens Sam with a gun and at the same moment Wong does the same to him.
- The Mole: The two leads are the most obvious example. Shen is an undercover cop, and Billy acts as a mole for the triads.
- Mortal Wound Reveal: Keung in the car after the shootout.
- My Greatest Failure: In the beginning of the second film, Wong tells Sam how he failed to save his partner from being impaled by a gangster during his junior year.
- Napoleon Delusion: In the third film, Ming turns out to have gone insane and believe that he is now Yan.
- No Good Deed Goes Unpunished: Invoked by the story of Wong told to Sam in second film, where he tried to stop a gang fight as a junior cop with his partner. His partner was mortally wounded by a gangster and Wong wounded and arrested the assailant in return. Wong encountered the said gangster nine years later, boozing with Ngai's gang members, as if he faced no consequences for his crime. Wong remarked the incident with a fitting proverb while voiced his regret for not emptying his gun in the gangster's face.
- No, Mr. Bond, I Expect You to Dine: In a morally-inverted example, Wong likes doing this to Sam whenever he's arrested.
- Oh, Crap!: Wong, when Del Piero manages to halt the elevator he's in.
- Sam in several instances, most notably him realizing that Lau is no longer on his side and has him cornered, just before he gets shot by him.
- One-Steve Limit: Exception with the two Marys.
- Out with a Bang: In the second film, Ngai's capo Gandhi is suffocated with a plastic bag by a female assassin while having sex with her.
- Overt Rendezvous: Hon Sam and Lau meet at a theatre.
- The Password Is Always "Swordfish":
- The password is the Morse code for "undercover".
- Lau leaves a message for his girlfriend that he stored Yan's file on his computer secured with her birthday as the password.
- Pretty Little Headshots: The entrance wound on Yan's forehead is tiny to allow for family-friendly close-ups on his head.
- The Purge: The massacre of Ngai's lieutenants in the second film, and Ngai's family near the end of the movie.
- Redemption Rejection: Lau doesn't want to be a gangster anymore, but the crimes of his past are not forgiven. Even as he manages to kill Sam in hope to erase all evidence that he worked for the triad, Yan finds the cassette that clearly proves that Lau has worked for Sam. That leads to Yan trying to arrest Lau, which in turn leads to Yan being killed by Inspector B. Yan's death then is the reason for the things happening in the third film, leading to Lau being defeated by Yeung and Cheng.
- Revised Ending: The mainland China, Malaysia, and Singapore version of the first film ended with a different ending in which Ming gets caught, because of government Moral Guardians wanting the message that crime does not pay.
- Rooftop Confrontation: Yan and Ming after they learn of each other's identity. Lampshaded by Ming:"Do all undercover cops like rooftops?"
- The Scapegoat: Billy. Ming pins everything on him at the beginning of the third film.
- Shoot the Hostage Taker: Billy shot Yan in the head while the latter was holding Ming at the gunpoint, before Billy revealed himself as another Sam's mole to Ming.
- Happened in the prequel too, this time with Wong shooting Hau while the latter was grabbing Sam.
- Sinister Shades: Wong has them most of the time. Also Shen in the third film.
- Spy Cam: Yan gets a pinhole camera hidden in a watch. It's not plot-relevant though.
- Survivor Guilt: Ming undergoes this following the events of the first film, which also drove him insane.
- Switch to Mandarin: Asked by Shen during his meeting with Yan and Yeung when they speak in Cantonese. It doesn't work; as Mandarin is the official language of mainland China and Shen comes from there.
- Those Two Guys: Del Piero and Keung are usually seen with each other in the scenes they appeared in the first movie.
- Through the Eyes of Madness: Most of the third film.
- Trapped Undercover: Yan, after Wong is killed and Ming erases his police file.
- Unperson: Ming erases Yan's police file after Yan discovered Ming's double identity as Sam's mole.
- Villain Protagonist: Lau Kin Ming for the whole series.
- Would Hurt a Child: Sam ordered his Thai partner to dispose all of Ngai's remaining family members, including his kids.
