All cop movies -- at least all the really fun, pulpy ones -- are about the thin line between cop and criminal. But this diabolically clever movie, the apotheosis of the Hong Kong police thriller, plays with that theme better than any film I've ever seen. There's a cop (Tony Leung) who's gone deep undercover as a mole in a local crime organization, and -- in a twist that the Lethal Weapon and Die Hard movies can only admire from afar, no touching -- there's a gangster (Andy Lau) who's gone deep undercover as a cop in the H.K. police force. And now, in a bit of plot ingenuity so crafty that it should probably be illegal itself, each of these men has been assigned by his unwitting boss to root out the other. Accolades enough cannot be heaped upon Leung and Lau for their slyly mesmerizing performances as two men who, as the cliché goes, are not so different from each other, filled to bursting with overdeveloped senses of honor and a love of the game even when it gets them into deep trouble. And kudos to director Andrew Lau (not the same guy as actor Andy Lau); his codirector, Alan Mak, wrote the film with Felix Chong (word is production started with an actual completed script, apparently a rarity in the Hong Kong film world): they layer the pulp with sensitivity and shades of gray, rather surprising and unexpected for a slam-bang shoot-'em-up cop flick, while keeping the tension and the suspense almost unbearably delicious. It's so good, in fact, that Hollywood is barging in to steamroller the steely delicacy and to mush the grays into hard-edged black-and-white. Matt Damon and Leonardo DiCaprio will be starring, which sounds like a joke and sadly is not. Happily, there's still IA 2 and IA 3 to be devoured on imported DVDs.