The Joker was an anarchist who terrorised the city of Gotham dressed as a psychotic clown. His calling card was a joker playing card, and he used his sick sense of humour to add comedy to all of his terrible crimes and misdeeds.
The Joker related two stories on how he developed his facial scars. In the first, he told of an abusive alcoholic father who beat his mother. One day his mother took a kitchen knife to defend herself, but he took the knife and attacked the mother. He then came at the Joker and stuck the blade in his mouth, and gave him a Chelsea Grin.
In the second story the Joker mentioned his wife, who was beautiful but had a gambling problem. When she got into debt, the loan sharks carved her face, and she became very miserable. In an effort to make her smile again, the Joker stuck a razor in his mouth and cut his own face, so he would be like her. She couldn't stand the sight of him and left, leaving the Joker always smiling.
After Batman became known amongst the criminals of Gotham, the Joker began making his presence known. He robbed mob banks, always leaving a joker playing card at the scenes of his crimes. One such bank was the Gotham National Bank, which he robbed with a group men dressed as clowns which he ordered to kill each other. He made off with the money in a school bus.
After stealing from the mob, the Joker visited one of their meetings, between Sal Maroni, the Chechen, and Gambol. Mr Lau was proposing leaving their remaining money with him, but the Joker said the only solution was to kill the Batman. He said he would do this, in return for half of their money. Gambol responded by putting out a bounty on the Joker's head.
Later, the Joker pretended to be dead and had himself taken to Gambol. One of his men asked for the bounty, as the Joker rose from his supposed grave and stuck a knife in Gambol's mouth. He told Gambol the story of his abusive father, before slicing his face. He then gave a broken pool cue to Gambol's men, telling them to fight over a position in the Joker's army.
Next, the Joker kidnapped Brian Douglas, one of the Batman impersonators. He made a video where he threatened to kill people unless Batman revealed his identity, before brutally murdering Douglas. He then strung Douglas's body up outside Mayor Anthony Garcia's window. Later he planted a bomb in judge Janet Surrillo's car, poisoned commissioner Gillian Loeb's glass and interrupted a fundraiser for Harvey Dent. When Dent was nowhere to be found, he approached Rachel Dawes, Dent's girlfriend, and told her story of his ex-wife who got into gambling debts. Suddenly Batman appeared and attacked the Joker, who responded by holding Rachel hostage and throwing her out of the window. As Batman rushed to save Rachel, the Joker made his escape.
The Joker then killed two men named Harvey and Dent, leaving a paper behind with the Mayor's face circled as his next threat. At the memorial for commissioner Loeb, the Joker and his men took the clothes of the men firing the salute and shot at the mayor. In the chaos the Joker got away, and although the Mayor survived, police lieutenant James Gordon was apparently killed.
Harvey Dent then revealed himself as the Batman, and the Joker pursued his police escort in an attempt to kill him. He diverted the convoy using a burning fire engine, and then shot at the police van using a shotgun and bazooka. However the real Batman showed up in The Tumbler, and saved Dent from the Joker's missiles. The Joker then took the wheel of his truck and faced Batman, who flipped his truck but crashed when unable to kill the Joker. The clown then drew his knife and was about to attack Batman when James Gordon, who had faked his own death, appeared and arrested the Joker.
Meanwhile, the Joker used his influence to have Harvey Dent and Rachel Dawes kidnapped and put in warehouses containing barrels of oil. After being interrogated by Batman, he revealed the two locations - but also mentioned that they only had time to save one person. He then talked his guard into attacking him, and managed to hold him hostage with a piece of broken glass. He then demanded a phone call, and used it to detonate a phone bomb he had surgically implanted into one of his thugs. He then escaped from the MCU, after freeing Lau and taking him with him.
After Coleman Reese threatened to reveal the true identity of Batman, the Joker called for his execution or a hospital would be blown up. He then visited Gotham General dressed as a nurse, and confronted Harvey Dent. He told Harvey to embrace chaos, as it was the only fair thing in a corrupt world. As he left, he blew up the hospital. He then made Lau give up the rest of the mob's money, and burnt it all. He then killed the Chechen and took his dogs.
The Joker then kidnapped Mike Engel and made him reveal that everyone had to get off the Island or they would die. He had laden two ferries full of explosive, and each ferry had to decide whether the other would survive. Batman located the Joker and attacked him, capturing him with a rope. Although neither ferry exploded, the Joker revealed his true plan: Harvey Dent had been driven insane, capturing James Gordon's family. The Joker told Batman that they were destined to fight each other for ever, before he was arrested and taken away.