I'm not big on cop shows. Got really bored of the genre. However "Infernal Affairs" was just superb. Actually this is a whole trilogy out of Hong Kong. After "Infernal Affairs I", you get a prequel "Infernal Affairs II" and you finally get the sequel "Infernal Affairs III". I've seen I & III and thoroughly enjoyed both. No doubt the first movie is the best but III was worthwhile. Can't wait to get my hands on II.
The basic plot of I is about two moles. One mole works for a triad and has infiltrated the police. Another mole works for the police and has infiltrated the triad. Both groups know a mole exists in their own organsiation and both moles are tasked on finding the other.
There are plenty of twists to the story and the narrative is very clever with the way it jumps around the story timeline. Along with dark and moody cinematography and excellent character acting, the film really builds up the tension and propels you along goading you to figure out what's going on before yet another twist is beset upon you. It's like a classic film noir thriller and miles away better than the shout, chase and shoot Hollywood cop drama style.
Highly recommended. I really hope Hollywood doesn't try and remake it.