Infernal Affairs

Infernal Affairs is a 2002 Hong Kong crime-thriller film directed by Andrew Lau and Alan Mak. It tells the story of a police officer who infiltrates a triad, and another officer secretly working for the same gang. It is the first in the Infernal Affairs series and is followed by Infernal Affairs II and Infernal Affairs III.
The Chinese title means "The Unceasing Path", a reference to Avici, the lowest level of Hell in Buddhism, where one endures suffering incessantly. The English title is a word play, combining the adjective 'infernal' (concerning hell) with internal affairs – the police department concerned with investigating its own officers.
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 and gave it a limited US theatrical release in 2004.

Plot

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. 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.
Using Morse code the Chan is able to relay data back to the police. However, Lau alerts Hon, giving him enough time to order his minions to dispose of the evidence. After the incident, both Wong and Hon are tasked with finding the moles in their respective organization.
Wong intends to pull Chan out of undercover work for fear of his safety. However, he 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.
Through this incident, 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. However, back at the police headquarters, Chan discovers that Lau was the mole and leaves immediately.
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.