Its an American remake of a Chinese movie...

Rating: 8/10 (4 November 2006)

The departed is a remake of a Chinese movie called "Mou gaan dou" or "Infernal Affairs" for its American title. I give the original 8 stars, and highly recommend seeing it. I have yet to see "The Departed", but plan on checking it out to see if it is as good as the original Chinese version.
Basically the mafia and the police both send a mole into the others organization to try and thwart the others plans. One to try to help catch the mafia lord.. the other to keep the police from catching his boss. Neither knows who the other is, but eventually begin trying to find out one anothers identities.