From NetFlix:
To take down South Boston’s Irish Mafia, the police send in one of their own to infiltrate the underworld, not realizing the syndicate has done likewise in Martin Scorsese’s multiple Oscar-winning crime thriller, including Best Director and Best Picture. While an undercover cop (Leonardo DiCaprio) curries favor with the mob kingpin (Jack Nicholson), a career criminal (Matt Damon) rises through the police ranks. But both sides soon discover there’s a mole among them.
“1001 Films to See Before You Die” praises Scorsese for successfully adapting a successful Hong Kong action film “Infernal Affairs” and its two sequels to the Boston Irish-American culture. For this film Mark Wahlberg was nominated for “best actor in supporting role”. Needless to say it is violent and not for children.