Infernal Affairs is a 2002 Hong Kong action thriller film co-directed by Andrew Lau and Alan Mak. Jointly written by Mak and Felix Chong, it stars Andy Lau, Tony Leung, Anthony Wong, Eric Tsang, Sammi Cheng and Kelly Chen. The film follows an undercover Hong Kong Police Force officer who infiltrates a Triad, and another officer who is secretly a spy for the same Triad. It is the first in the Infernal Affairs series and is followed by Infernal Affairs II and Infernal Affairs III.

Plot

Hon Sam, a Hong Kong Triad boss, sends Lau Kin-ming, a young gangster, to the police academy to serve as his spy in the Hong Kong Police Force. Around the same time, Chan Wing-Yan, a young police cadet, is seemingly expelled from the police academy. In reality, Chan has secretly become an undercover cop, reporting only to Superintendent Wong Chi-shing, who sends him to infiltrate Hon's triad. Over the course of ten years, Chan experiences great stress from his undercover work while Lau quickly rises through the ranks in the police force, eventually becoming a Senior Inspector. Wong and his team interrupt a deal between Hon and a group of Thai cocaine dealers after receiving a tip from Chan. However, Lau alerts Hon, giving him enough time to get his henchmen to dispose of the evidence.