Movies You Might Have Missed: Infernal Affairs

Andy Lau is a cop who is working on the force as a mole for a big time triad boss played by Eric Tsang. On the other side Tony Leung plays a cop under deep cover as a mole in Tsang's organization. Each side becomes aware of the existence of the other's mole during a drug deal and an attempted bust. The deal fails, but so does the bust and both sides realize their failure was due to the presence of a spy in their midst.