I didn't get The Killing Joke!?

The plot revolves around a largely psychological battle between Batman and his archnemesis the Joker, who has escaped from Arkham Asylum. The Joker intends to drive Gotham City Police Commissioner James Gordon insane to prove that the most upstanding citizen can go mad after having "one bad day." Along the way, the Joker has flashbacks to his early life, gradually explaining his possible origin.