Directed by Andrew Lau and Alan Mak, Infernal Affairs is a tense thriller featuring Hong Kong superstars Andy Lau and Tony Leung. The film follows the parallel lives of Ming (Lau), a cop who secretly reports to ruthless Triad crime boss Sam (Eric Tsang); and Yan (Leung), an undercover police officer who poses as a Triad member in Sam's gang. For Yan, the years of living in the criminal underworld have taken their toll, and he longs to return to regular police duty. However, the only person who knows Yan's true identity is his mentor Superintendent Wong (Anthony Wong), also the unsuspecting superior of Ming. When Wong's officers come face to face with Sam's gang, both leaders realize there are moles in their midst. Soon Yan and Ming must track each other down, leading to an inevitable confrontation.

Unlike many contemporary Hong Kong films, Infernal Affairs steers clear of over-the-top action in favor of a more stylized and subdued story that builds on emotional and psychological tension.