In late Seventies New York, FBI agent Joe Pistone (Johnny Depp) has to go undercover to infiltrate the Mafia as 'Donnie Brasco'. He befriends ageing hitman Lefty Ruggerio (Al Pacino), who inducts him into the ways of the Mob. As Brasco, Pistone becomes immersed in the Mafia world of violence, but at the same time comes to regard Lefty as a friend. He knows that when his assignment is over and his cover blown, it will mean certain death for the man who has taken him under his wing.