I can't believe there hasn't been a topic for this yet, but here we go...
This movie opens not long after Batman Begins, and details how Batman's nemesis, the Joker, brings Gotham City into anarchy as Batman and his more legitimate allies, police officer Gordon and district attourney Harvey Dent battle against him and the mob that hired him.
In general, the movie is well-plotted and generally surprising, though there are a few scenes that are a bit odd and the Joker seems to be able to teleport explosives into whatever space the plot needs them to be. Speaking of which, the Joker is pretty much the star of the show, overshadowing even Batman. He comes off as a twisted, disturbed, ingenious anarchistic and I found myself wishing he appeared more often. A particularly excellent scene involves the Joker breaking into a mobster meeting and preforming one of the most entertaining magic tricks I've ever had the chance to see.
Anyways, before this grows into a review of novelistic proportions, I'm going to finish this up and simply say: go see it.