Originally Posted by JamesK

Not really underrated, but I think because it's a Chinese movie, not many people from the western civilization know about it. It's one of my favorite movies.

Mou gaan dou (Internal Affairs)

Shawn Yue plays a young police officer who has been sent undercover as a mole in the local mafia. Edison Chen plays a young mafia member who is infiltrating the police force. Years later, their older counterparts, Tony Leung and Andy Lau respectively, race against time to expose the mole within their midsts and destroy him.

http://www.imdb.com/title/tt0338564/