This is the true story of Joe Pistone(Johnny Depp), who went uncercover to collect evidence on the New York mafia in the 1970's. Lefty, played by Al Pacino, takes him under his wing. His wife (Anne Heche) has difficulty understanding how work could be so important and why he is unable to talk about it. Anyone who has seen this movie will remember, "Forget about it.
One point of the story is clear: do whatever it takes to catch the bad guys. However, Pistone really becomes Donnie Brasco, his undercover persona. He know that his mob mentor will die when he is revealed. They have developed a strong friendship and he doesn't want to betray the man who vouched for him. So we have another point: friendship. Or you could interpret it as a lesson about what happens when you run with the wrong crowd, like my mother always warned me.
In the end, the FBI pulls Pistone, gives him a medal and a $500 reward and sends him into their version of the witness protection program. Surprisingly, he manages to stay with his wife, who was undersandably extremely upset by his three year foray away from home. Chalk up a few points for the power of love, too.