A series of disasters drives the government to completely abandon a city and those inhabitants who can't/or won't run away to fend for themselves. The people who remain are the poor, the unlucky, those who see an opportunity for power and control outside of societies bounds and oh those few people who are willing to put themselves through everything to defend the former against the latter. It's interesting, deep, provoking and does a good job of introducing every character involved fully and showing them as they are, without depending on prior experience. As a result it's worth reading even if your only experience of Batman is the two Nolan films (and I'd played Arkhum Asylum, but that meant I had an over abundance of knowledge!). I started reading it at 11pm and before I knew it, it was 4am and I only had a few pages left. Even now it's over I'm still thinking about the themes and questions it raises.
Saying all that, there are flaws.
1. Some parts of the story are told in third-person personal with the Joker. I don't know much about the Joker, but to me the specialness of him has always been the way he's completely unleashed from everything. Morals, respect, society, reality. You cannot control him or predict what he'll do next. By all means have him force 13 thugs to play a game of baseball and casually beat one to death with a baseball bat because there are too many, but don't tell us what he's thinking while he does it. It makes him understandable
2. The story is told one level above what you should expect, there isn't much focus on the suffering of the civilains or the on the ground day-to-day plight of the city, instead it's purely about the higher level between heroes and villains.
3. It's a bit compressed, a story as grand as this needs a Lord of the Rings epic, it's the rise and fall of the city. Sometimes everything feels a little too easily resolved and it even feels like the master plan at the end was just things falling together.
But those are nitpicks, this story focuses on the basis of civilisation and who people are. Batman isn't a hero because of what he can do, but because he will stop at nothing to save other people and give up everything he has in sacrifice. As Batman he has to be more than human. As Bruce he has to act as someone he isn't. There's no room for who he is