Infernal Affairs

Chan Wing Yan (Tony Leung) is a hard-nosed veteran cop sent undercover to infiltrate the notorious Triad crime ring. An expert at bringing down violent syndicates, Chan thinks it's going to be a routine mission. What he's not prepared for is the discovery that the Triad's boss (Eric Tsang) has planted a mole (Andy Lau) in the police department ... and now Chan is being hunted down. In this battle of wills, only one cop can win! (official distributor synopsis)