Dateline Boston. Damon is a young hood in the Irish mafia who infiltrates the Police Department as a cadet. DiCaprio is a cop who goes undercover as a hood in the Irish mafia. Then, when both have their identities revealed, things get really tricky. Jack Nicholson, Martin Sheen and Mark Wahlberg
http://thedeparted.warnerbros.com/
Awesome movie, DiCaprio was amazing, Nicholson was incredible, the story was original with great writing. Produced by Martin Scorsese.
There have been alot of shit movies, finally one I can rave about and leave saying that was a great movie.
Go see it!