The final installment (to date) of the Infernal Affairs trilogy eschews operatic shootouts in favor of following the undercover inferno to it's inevitable identity meltdown. The final film is so full of moles on either side it's hard to tell at times if the majority of gangsters in Hong Kong aren't additionally claiming a modest law enforcement wage, and visa versa. It's also pretty hard to keep track of who's getting paid by both sides and who isn't, and even characters who are getting paid in this one may be donating their wages to orphanages.
The big story here is how Triad police infiltrator Inspector Lau (Andy Lau) can come to terms with his now apparently massive dependency on Chan (Tony Leung), and his desperate attempts at redemption. Lau's identity hemorrhage is mapped across a series of flash backs, as once again the action skips from pre- to post-handover Hong Kong. Dr. Lee Sum Yee (Kelly Chen), the least convincing psychiatrist in the history of cinema, makes an unwelcome comeback from Infernal Affairs I. The plus side of this is that we get to see Chan at his most charming, fooling around, flirting, eating and sleeping through his state required sessions. To be honest, the film was worth making just to expand on Chans role and get a bit more Leung screen time.
It's not the greatest film in the world and doesn't really make much sense unless you've payed attention to I and II. But it does give audiences what they want - more scenarios around the established characters, chucks in a couple of new comers to keep things interesting, and doesn't go (completely) out all guns blazing predictably.