IThe story began with Batman discovering that the Joker had escaped from confinement within Arkham Asylum, who was later seen striking a deal with a man for a seemingly forsaken carnival site. The Joker then proceeded to making a visit to Commissioner Gordon's abode, where daughter Barbara Gordon was shot through the spine by the man himself, which led to her state of paralysis down the waist, before kidnapping the Commissioner.
Before the Joker, he was portrayed as an out-of-luck comedian bearing a pregnant wife, struggling to make ends meet, before resorting to the way of crime under the guise of the Red Hood.
The Killing Joke also illustrates the Joker's twisted sense of the way the world works through torturing Commissioner Gordon, blatantly mocking the conventional perception of order and sanity. In a fiendish way, he was emulating on the Commissioner whatever the man had supposedly once been through that left him in the state he was then, trying to vindicate that anyone, with the right amount of force, could be pushed off the brink of sanity.
The Joker goes on further as to point out that Batman was no more insane than he was, to be dressed up as a 'flying rat' for instance, something people would not regard as a sign of normality under any circumstances; only that unlike the Joker, he tried to pretend that everything in the world still makes sense, that there's still something out there worth fighting for, or waiting for such an occasion rather.