The Killing Joke is one of the most popular Batman stories ever. Mastermind Alan Moore wrote this one-shot story to give a more detailed origin of Batman‘s worst enemy, the Joker. Also this story provides why Barbara Gordon becomes paralyzed and can‘t be Batgirl anymore, which led her to become Oracle.
There are actually two plots in The Killing Joke. The man who will become the Joker is an ordinary guy. He works as an engineer and has a pregnant girlfriend Jeannie. We don‘t get to know his real name. The engineer is desperate about his job, he wants to quit and become a comedian instead (where is failing, too). So he meets two guys who want to rob the chemical plant where the Joker worked previously to his attempts to become a comedian. They meet in a bar to talk about the details of their robbery. Soon two police officers enter the bar and walk straight to the threesomes table. They tell the engineer that his girlfriend died in a household accident. In his desperation he is willing to help the criminals.