That's what he is, I guess: a disease that infected Gotham City...
...of which there is no cure.
The Joker is inexplicably released from Arkham and he quickly discovers that he's broke. And well, he might be insane, but he's not crazy enough to let people get away with that. The Penguin, The Riddler, Two Face, etc. are to blame. As far as he's concerned anyway.
With the help (at least presence) of his new henchman, Johnny Frost, Joker sets off to get his property back, to get a little revenge, and generally wreak havoc. You know, as you would if you were the Clown Prince of Crime.
That's really all I'm going to say—it's bloody, it's depraved, it's dark, it's twisted. It's not revolutionary, it's not a reinvention of the Joker. It's a good story about the character that's been a favorite of readers for decades.
Oh, sure, the Dark Knight puts in an appearance—but it's at the end, and he's not even brought up for most of the book. Then he's there and things go the way they usually do when he shows up.
The art? Hoo-boy. It's something else. It's...visceral is the best word I can come up with it. Even if you don't like it, I don't see where you can't have a strong opinion of it—I find it striking, memorable...and visceral.
I really enjoyed this one and highly recommend it to any comic reader, or anyone who wants to see what the world of Batman can look like in the original medium instead of film.