The story starts with the Joker being released from Arkham Asylum, and he quickly discovers that others have taken over his territory. Moving quickly we see the Joker begin to take back what he believes is his, in his own violent and twisted fashion. We even get to see Azzarello's take on the eternal battle between Batman and the Joker.
A quick read, this is a great noir tale that Azzarello tells so well, the characters would not seem out of place in 100 Bullets. Well worth reading.