Pre-release publicity focused on its star-studded cast (Tony Leung, Andy Lau, Anthony Wong, Eric Tsang, Kelly Chen and Sammi Cheng), although the film also received critical acclaim for its original plot and its concise and swift storytelling style.
The film had been selected as the Hong Kong entry for the Best Foreign Language Film at the 76th Academy Awards but it was not nominated. Miramax Films acquired the United States distribution rights of this film and gave it a limited US theatrical release in 2004.
Chan Wing-yan, a police officer, goes undercover into a triad. Around the same time, Lau Kin-ming, a triad member, infiltrates the Hong Kong Police Force. Each mole has been planted by the rival organisation to gain an advantage in intelligence over the other side. The more the moles become involved in their undercover lives, the more issues they have to cope with.
The prologue opens with triad boss Hon Sam sending a number of young gangsters to the police academy as moles, among whom is a young Lau. Concurrently, a young Chan joins the police force but is seemingly expelled from the academy even though he manages to impress Superintendent Wong Chi-shing. In reality, Chan has become an undercover agent reporting only to Wong. Over the course of ten years, Chan experiences great stress from his undercover work while Lau quickly rises through the ranks in the police department. The film begins with a meeting between Chan and Lau in a hi-fi store without either of them knowing the other's true identity.
Wong and his team interrupt a deal between Hon and a Thai cocaine dealer after receiving a tip-off from Chan using Morse code. However, Lau alerts Hon, giving him enough time to order his minions to dispose of the cocaine, eliminating solid evidence of the drug deal. After the incident, Wong and Hon both know that they each of them has a mole within their respective organisations, placing them in a race against time to root out the other mole. Lau is tasked with finding the mole in the police. Later, Chan sees Hon conversing with Lau in a cinema but does not see Lau's face clearly. He tries to follow Lau and find out who he is, but gets distracted by a phone call. By this time, both Chan and Lau are struggling with their double identities – Chan starts losing faith in himself as a cop after being a gangster for ten years; Lau becomes more accustomed to the life of a police officer and he wants to erase his criminal background.
At their next meeting, Wong intends to pull Chan out of undercover work for fear of his safety. They are unaware that Lau has his subordinate, CIB Inspector B, tracking him. Lau relays Wong's location to Hon, resulting in Hon's henchmen being sent to confront Wong. Lau relays Wong's location to Hon, who sends his henchmen after Wong. Inspector B informs Lau of the henchmen's arrival, and an OCTB squad is dispatched to save Wong. Chan successfully flees from the building; Wong is caught by Hon's men and is killed when he is thrown off the building, having refused to reveal Chan despite the beating from the gangsters. As the police close in, a shootout ensues in which several gangsters are killed. Keung drives Chan away from the scene, but later dies from a gunshot wound. It is reported on the news that Keung was actually an undercover cop; Hon assumes that Keung was the mole and that Chan killed him to protect the triad.
Lau retrieves Wong's cell phone and contacts Chan; both of them agree to foil a drug deal by Hon. The plan succeeds and many of Hon's men are arrested, while Lau betrays Hon and murders him. Everything seems to have returned to normal – Chan can revert to his true identity as a cop, while Lau has erased his criminal connections by eliminating Hon's triad. However, back at the police headquarters, Chan discovers that Lau was the mole and leaves immediately. Lau, realising what has happened, erases Chan's file from the police database. Chan spends an evening with his psychiatrist, Lee Sum-yee, with whom he has fallen in love. He sends to Lau a compact disc containing recordings of phone calls between Hon and Lau. Lau's fiancée, Mary, plays the disc, finds out about Lau's criminal connections, and expresses her disappointment with him.
Chan and Lau meet on the same rooftop where Wong was killed earlier. Chan disarms Lau and holds a pistol to his head as a rebuke to Lau's plea for forgiveness and request to remain as a cop. Inspector B arrives on the scene shortly and orders Chan to release Lau. Chan holds Lau as a hostage at gunpoint and backs into the lift, but upon moving his head from behind Lau he is suddenly shot in the head by B. B then reveals to Lau that he is also a mole planted by Hon. As they take the lift down to the lobby, Lau kills B out of his desire to eradicate traces of his past, become a "good guy" cop, and end the mole hunt.
Stepping out of the lift, Lau shows his identity card to the police to identify himself as one of them. Months after Chan's death, Lee discovers records revealing Chan's true identity as an undercover police officer; B becomes a scapegoat for Lau as the real mole in the police force and the case is closed. Lau salutes Chan at his funeral. A flashback reaffirms the point that Lau wished he had taken a different route in life.
In mainland China, an alternative ending for the film was created. Lau exits the lift and is informed by the police that they have found evidence that he was a mole. Lau hands them his badge and is arrested without protest. The sequel, Infernal Affairs III, uses the original ending instead of the alternate one.Tony Leung as Chan Wing-yan (陳永仁), an undercover cop in Hon's triad.
Andy Lau as Senior Inspector Lau Kin-ming (劉健明), Hon's mole in the police force.
Edison Chen as young Lau Kin-ming
Shawn Yue as young Chan Wing-yan
Anthony Wong as Superintendent Wong Chi-shing (黃志誠), Chan's superior.
Eric Tsang as Hon Sam (韓琛), the triad boss and main antagonist.
Chapman To as "Silly" Keung (傻強), Hon's henchman.
Gordon Lam as Inspector B (大B; Big B), Lau's subordinate who is also a mole in the police force.
Kelly Chen as Lee Sum-yee (李心兒), Chan's psychiatrist.
Sammi Cheng as Mary, Lau's fiancée.
Berg Ng as Senior Inspector Cheung (張Sir), Wong's subordinate.
Wan Chi-keung as Officer Leung (梁Sir), the police chief.
Dion Lam as Del Piero, Hon's henchman.
Elva Hsiao as May, Chan's ex-girlfriend.
Infernal Affairs was released on 12 December 2002 to Hong Kong Theaters to universal critical acclaim. Review tallying website Rotten Tomatoes reported that 57 of the tallied 60 reviews were positive, for a score of 95% and a certification of "fresh". Infernal Affairs won seven out of the sixteen awards it was nominated for at the 22nd Hong Kong Film Awards, beating Zhang Yimou's Hero for the Best Film award. It also won Best Picture awards in the Golden Horse Awards and the Golden Bauhinia Awards among other awards too. It was ranked No. 30 in Empire magazine's "The 100 Best Films of World Cinema" in 2010. It is the highest ranked Hong Kong film on Internet Movie Database's Top 250 movies list.
Infernal Affairs has grossed HK$55,057,176 in Hong Kong and USD$169,659 in North America.
The original film score for Infernal Affairs was written and performed by Chan Kwong-wing.
The theme song, Infernal Affairs (無間道), was composed by Ronald Ng, lyrics provided by Albert Leung, and performed in Cantonese and Mandarin by Andy Lau and Tony Leung.
Although not included in the soundtrack, Tsai Chin's (蔡琴) song "Forgotten Times" (《被遺忘的時光》) features prominently in this film as a recurring element of its storyline, and also in its sequels. As well as serving to elucidate the theme of the films, the song plays an important plot function in chronologically connecting various elements of the story. The (a capella) song can be first heard when Chen and Lau meet in a store, as they are analyzing hi-fi equipment.
The success of the film followed with a prequel and sequel. An open-world video game titled Sleeping Dogs (or True Crime: Hong Kong before canceled by Activision Blizzard in 2011) was heavily inspired by Infernal Affairs, with the protagonist of the story infiltrating the criminal underworld as an undercover police. Sleeping Dogs was developed by United Front Games and published by Final Fantasy and Dragon Quest series owner Square Enix.
The movie was listed on LOVEHKFilm's Top 50 Movies of the Decade.
A TV series of the same name was commissioned for HKTV in 2016 and will air for three seasons, with an entirely new cast and also returning characters from the films.
In 2003, Brad Pitt's production company Plan B Entertainment acquired the rights for a Hollywood remake, named The Departed, which was directed by Martin Scorsese, and starred Leonardo DiCaprio, Matt Damon, Jack Nicholson, and Mark Wahlberg, set in Boston, Massachusetts, roughly based on the life of famed Boston mobster James "Whitey" Bulger. The Departed was released on 6 October 2006 and won the Academy Award for Best Picture.
Lau, Tsang, and Cheung parodied the cinema scene to promote the Hong Kong Film Awards. Lau and Tsang, in their respective characters, go through the scene where they meet to gather info on the undercover cop amongst Hon Sam's gang. Lau Kin-ming asks Hon "Why do we always meet in a cinema?", to which Hon answers "It's quiet. No one comes to movies". Cheung comes out from the shadows behind them and says "I don't know...quite a few people watch movies" and we see a slew of Hong Kong celebrities watching various clips of Hong Kong films on the screen. Originally Tony Leung was going to appear but scheduling conflicts led to the recasting.
The 2003 TVB spoof celebrating the Chinese New Year called Mo Ba To (吐氣羊眉賀新春之無霸道), the 2004 comedy film Love Is a Many Stupid Thing by Wong Jing, and the 2004 TVB television drama Shades of Truth were re-writings based on the plot of the film.
In Taiwan SHODA (劉裕銘) and a secondary school student Blanka (布蘭卡) cut and rearranged the original film and inserted new sound tracks to produce their videos Infernal Affairs CD pro2 and Infernal Affairs iPod on the web. The videos had many views and both producers removed their videos after receiving cease and desist letters from the Group Power Workshop Limited (群體工作室), the Taiwan distributor of the film.
Media Asia released a limited edition of eight-DVD set of the Infernal Affairs trilogy in an Ultimate Collectible Boxset (無間道終極珍藏DVD系列(8DVD套裝)) on 20 December 2004. Features included an online game and two Chinese fictional novels of the film series by Lee Muk-Tung (李牧童), titled 無間道I+II小說 ISBN 962-672-259-2 and 無間道III終極無間小說 ISBN 962-672-271-1.
The hi-fi shop scene was later recreated with additions of excerpts of the film to encourage businesses to join the Quality Tourism Services Scheme in Hong Kong.
In 2009, a Korean remake City of Damnation, which was directed by Kim Dong-won was released on 22 January 2009. In 2009, a Telugu remake Homam, which directed and acted by JD Chakravarthy along with Jagapathi Babu was released and became a notable movie. In 2012, Double Face (ダブルフェイス), a Japanese television remake starring Hidetoshi Nishijima was released by TBS and WOWOW. The production aired in two parts: "Police Impersonation" on WOWOW and "Undercover" on TBS.