Infernal Affairs
無間道 (original)
A veteran cop goes undercover to expose a detective who leaks information to the criminal underworld, but the mole doesn't know who's tracking him.
Starring Andy Lau, Tony Leung Chiu-wai, Anthony Wong, Eric Tsang, Kelly Chen, Sammi Cheng, Edison Chen, Shawn Yue, Chapman To, Dion Lam Dik-On
Watch "Infernal Affairs" on Criterion Channel released in 2002. It's a action & adventure and crime movie, and has a rating of 8 out of 10 from Imdb which is usually fine for this kind of movie.