Sequels are often a bad idea. If a second story is integral to our understanding of the first, it would have been included within it. Often, sequels seem like a cheap way to extract more life out of popular characters, by forcing the through fresh adventures which they either do not fit without contrivance, or which merely copy their previous escapades. 'Internal Affairs 2', however, is an exception. The first movie in this series was a complex thriller that was presented as the end game in a long battle between the Hong Kong police and criminal gangs; but the back story was only hinted at. This movie, actually a prequel, tells tells that story in such a way that it stands completely alone, and remains interesting although the audience already knows the ultimate ending; indeed, is arguably even more interesting because we know where the tale must end. One reason it works is because the film has different ambitions to its predecessor: that was a straightforward thriller of the highest order, whereas this film (no less good) is more character driven, and takes a wider perspective on Hong Kong society in general. Although the first movie was compared by some to Michael Mann's 'Heat', in fact it is this film that better bears the comparison as a tale of adversaries on opposite sides of the law, and it stands up to that comparison well: the subtle behaviours of the heroes and villains alike more interesting than the macho posturings of the gangsters and cops depicted in American movies. The only disappointment is the absence of Tony Leung from the original cast; but it's rare that two movies in a series are as complementary, and as good, as these two.