The film stars Andy Lau, Tony Leung, Anthony Wong, Eric Tsang, Sammi Cheng and Kelly Chen.

Miramax Films acquired the United States distribution rights and gave it a limited American theatrical release in 2004.

Around the same time, cadet Chan Wing-yan is ostensibly expelled from the police academy, but is actually selected to be an undercover cop reporting only to Superintendent Wong Chi-shing, who sends him to infiltrate Hon's triad.

Chan is afraid he is turning into an actual criminal while also fearing his cover will be blown; he begins undergoing sessions with psychiatrist Lee Sum-yee and jokingly tells her that he is a cop.

Henchman Keung, unaware that Chan is the mole, chauffeurs him away from the scene but succumbs to a gunshot wound he sustained earlier.

Lau states calmly that he wants to start over as a good person, but Chan rejects his plea to help him conceal his criminal past.

[5] In the original Hong Kong ending, Lau concealed his true identity as a triad spy and identified himself as a police officer to avoid punishment.

The script of Infernal Affairs was inspired by John Woo's Face/Off (1997) but Mak knew that its science fiction elements were too implausible in reality, so he focused on the exchange of identity and psychology between the two leads[7] as its themes.

[7][9] The script for Infernal Affairs originally belonged to Andy Lau's Teamwork Motion Pictures, but due to the company's entanglement in a lawsuit at the time, it was unable to produce the film.

[10] In addition, the creative team could not find investors because other studios at the time thought that an undercover film wasn't novel enough to make money.

Eventually, Andrew Lau made a hopeless bid and showed the script to John Chong at Media Asia Entertainment Group.

In Infernal Affairs, the identity crisis suffered by both Chan and Lau as moles hint at the struggle of Hong Kong residents, who faced both the colonization by the British and the reunification with Mainland China.

Specifically, under Deng Xiaoping's one country, two systems policy, the duplicity, unsettling, and uncertain nature of the future of Hong Kong residents is tightly echoed in Chan and Lau's character developments.

Scholar Howard Y. F. Choy further claimed that "this postcolonial (re)turn is actually more a recolonization than a decolonization of the capitalist Cantonese city by the mainland Mandarin master.

This concept of timeless, placeless, and endless suffering especially applies to the character Lau throughout the trilogy, who infinitely bears the burden of self-betrayal (serves as a mole), loss of family and friendship, and unsettledness.

"[15] New York Times reviewer Elvis Mitchell was so enraptured with the film that he stated that "Infernal Affairs is so beautifully shot that the images occasionally distract you from the condensed policier plot.

Composer: Ronald Ng Lyrics: Albert LeungSung by: Tony Leung, Andy Lau With star power, visual allure, and an engaging script, Infernal Affairs did very well critically and financially, spawned two sequels and a television series, and attracted the attention of Hollywood.

[18] 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, written by William Monahan, starred Leonardo DiCaprio, Matt Damon, Jack Nicholson, Mark Wahlberg, Martin Sheen, Ray Winstone, Vera Farmiga, and Alec Baldwin, set in Boston, Massachusetts, and roughly based on the life of famed Boston mobster James "Whitey" Bulger.

[21] 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 novelisations of the film series by Lee Muk-Tung (李牧童), titled 無間道I+II小說 and 無間道III終極無間小說.

[22][23] In 2012, Double Face (ダブルフェイス), a Japanese television remake starring Hidetoshi Nishijima was released by TBS and WOWOW.

[25] The story takes place years after the films' events, with some minor characters reprising their roles alongside a new cast.

The TV series uses the same concept as the film, but with an entirely new story and characters, and the setting expanded beyond Hong Kong to include Thailand and Shenzhen.

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.

[28] 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.

Although not included in the soundtrack album, Tsai Chin's 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.