A triad boss sends maybe half a dozen of his men, the ones with clean records, to join the police force and work as some kind of reverse undercover agents. At the same time that these men go into training, another recruit is singled out by the police to work undercover in the triads. In ten years, a triad mole in the police force has been promoted to a high position in the anti-triad bureau, and the police mole in the triad society is the right-hand man of the same boss who sent undercover gangsters into the police academy. When the mole in the police force is put on task force to take down his boss, both sides realize that they have moles in their ranks, and both of the undercover men are charged by their real bosses with rooting out the enemy mole. In other words, it gets complicated.
While most of these movies tend to wallow in boring melodrama and bad comic relief, Infernal Affairs goes the route of building suspense. It works fairly well, too. The complicated nature of the story actually helps a little, since keeping track of what's going on is enough of a challenge to hold the viewer's interest.
The cinematography is rather impressive as well. Nothing earth-shattering, but it's certainly above average.
Not that the film is flawless, but it is better than I expected.