Infernal Affairs is one of the most internationally recognized Hong Kong films, but the first time I watched it I was mightily disappointed. My head was too wrapped up in (very stylized) Japanese cinema back then, and this was one of the first Hong Kong films I watched purposefully. It was just too close to the type of Hollywood cinema I hated. Revisiting the film in the wake of The Departed did some good, but this will never be a personal favorite of mine.
Yan is an undercover cop who worked himself up in a big Triad organization. After ten years, he finally made it to where he wanted to be. Ming is a Triad mole who joined the cops when he was 18. With the help of his bosses, he was quickly promoted to a meaningful position. When Ming is ordered to arrest his Triad boss, the two moles will have to get very tactical if they don't want to blow their cover.
Leung and Lau are good but Wong and Tsang are better. The plot is fine, but not all that special, the cinematography (and certainly the editing) felt a little hasty. Infernal Affairs is a perfectly serviceable blockbuster filler, though I actually liked the sequels better. At least it wasn't bested by Scorsese's remake, but that's hardly praise coming from me.