One night Batman goes to visit the Joker in Arkham Asylum. While contemplating the futility of their relationship, and that it will only result in either his or the Joker's death in the future, he discovers that the Joker sitting before him is not actually the Joker, but an impostor. Batman threatens the imposter, and asks his infamous question "Where is he?!"
Meanwhile, the Joker is looking to buy a run-down old theme park for then unknown reasons. Through flashbacks, Moore reveals parts of the Joker's past, and shows he had a significant other named Jeannie who was pregnant with his child. Joker kills the owner of the theme park with his joker venom (the same one that made him the way he is) and makes the land his own. Later, Barbara Gordon answers the door only to find the Joker on the other side. The Joker shoots Barbara and takes nude photos of her on the floor and bleeding. His goons abduct Commissioner Gordon. In another flashback we see that the Joker was a failed comedian who, out of desperation for the life of his unborn baby, joins the mob for one job and becomes the Red Hood. Barbara is later revealed to be paralyses from the waist down. Batman consoles Barbara, and she tells him that this time its different. "The look in his eyes" she utters, while crying.
Commissioner Gordon is stripped naked and sent through an insane carnival ride, bombarded with the nude images of his daughter in an attempt to to break his sanity. He is ridiculed and made fun off by the Joker, to an audience of carnival freaks. Another flash back shows that before the Joker could even begin the job, Jeannie had died. Distraught, he has doubts about the job, but gets bullied into going through with it anyway. When Batman ruins the job, the Joker (while trying to escape) accidentally falls into a river of chemical waste and after removing his red helmet/hood, he sees what he has become. It becomes evident that he is insane, as he is shown laughing with blood dripping from his eyes and mouth. Back in the present, after gathering clues, Batman finds his way to the carnival and proceeds to fight the Joker and free Gordon. He tries to comfort Gordon, but is told to continue in the pursuit of the Joker, and to "bring him in by the books". The Joker attempts to shoot Batman with what would seem to be the same gun he used to cripple Barbara Gordon. The gun turns out to be a fake and Batman lives. The Joker tells an unusual joke about two insane men, and as he laughs uncontrollably, Batman utters a solitary chuckle. The story ends as it starts pouring rain and the police show up.
Some time later, she decides to become the omniscient computer presence known as the Oracle. She ends up being more useful and dangerous as the Oracle than Batgirl.
The Killing Joke has gained notoriety due to its labeling as a graphic novel, despite it's short length.
Tim Burton stated "I was never a giant comic book fan, but I've always loved the image of Batman and The Joker. The reason I've never been a comic book fan - and I think it started when I was a child - is because I could never tell which box I was supposed to read. I don't know if it was dyslexia or whatever, but that's why I loved The Killing Joke, because for the first time I could tell which one to read. It's my favorite. It's the first comic I've ever loved. And the success of those graphic novels made our ideas more acceptable." He has stated in the past that The Killing Joke was his inspiration for his Batman movie. There is however speculation as to whether or not this is a legitimate claim as Tim Burton's depiction of the Joker was much different to the one portrayed in the Killing Joke.