Donnie Brasco(1997)

In the 1970s, FBI undecover agent Joe Pistone infiltrates the mob, leaving his family behind and assuming the false persona of the "jewel man" Donnie Brasco. His assignment: to become a trusted insider with the infamous Bonanno family by gaining the confidence of a low-level gangster. Lefty Ruggiero is an aging, two-bit hit-man who sees a new future for himself with the smart, young thief Donnie Brasco and enlists him as his protege. Together the two men enter into a camaraderie that will not allow either one to distance himself emotionally. Meanwhile, Donnie begins to get lost in the distance between his real and undercover selves. As Donnie moves deeper and deeper into the Mafia chain of command, he realizes he is not only losing the line between federal agent and criminal, between who he pretends to be and who he actually is, he is also leading Lefty, his closest friend, to an almost certain death sentence.