See chapter 1.
He smiled into the dark when he looked at the clock and realized it as 11:53. His plan could be put into action soon. This time he wouldn't screw it up. He'd make Batman suffer like he never had before. He wouldn't leave him to bury his sidekick. No. Even if he died in the end, he would leave Batman with a constant reminder that he had existed and after years of fighting he had the last laugh after all.
He got up and left the old asylum. He started walking towards the spot where Harley and Arnold would be waiting. The Joker smiled as he walked through the alley and onto a fire escape. Just as he got there he saw Harley hit the boy with a mallet. He smiled as Arnold picked the boy up. "Now for the fun.