Joseph Pistone (Johnny Depp) is an FBI agent who has infiltrated one of the major New York Mafia families and is living under the name Donnie Brasco. He develops a relationship with mob hit man Benjamin "Lefty" Ruggiero (Al Pacino) in order to get deeper under cover, but ends up developing a real friendship with the Mafioso. As their relationship develops, Pistone must decide whether or not to complete his job, knowing that it will lead to the murder of his new friend.