This is the true story of the FBI agent Joe Pistone.

He infiltrated the mafia of New York. Befriending Lefty Ruggiero, Pistone (under the name of Donnie Brasco) was able to embed himself in a mafia faction lead by Sonny Black. Ruggiero and Pistone become tight as the group went about collecting money for 'the bosses'. Eventually, the group became big time when Black himself became a boss, all the while Pistone was collecting evidence. However, the trials and tribulations of the undercover work became more than Pistone could bear. His marriage fell apart.