The Joker is the main antagonist of The Dark Knight, described as a mysterious and maniacal anarchist psychopath portraying himself as an "agent of chaos", who rises to dominant power from the criminal underworld by terrorizing Gotham City and thrusting it into total insanity. His calling card is a joker playing card, and he uses his sick sense of humor and dresses as a psychotic clown to add comedy to all of his terrible crimes and misdeeds.
While very little information regarding the Joker's early life before he turned to a life of crime was confirmed, his true identity remained unsolved for the officials of Gotham City, and despite his capture, no traces could be found on his fingerprints, dental records, and DNA matches against the Gotham City Police Department's databases. The Joker's own testimony, while normally quite true when it came to carrying out threats, seemed at times contradictory, giving two conflicting accounts describing past events in his life.
One of these stories the Joker related on was that during his childhood, his abusive alcoholic father often beat his mother right in front of him. One night, when the father went off crazier than usual, the mother reacted by taking a kitchen knife to defend herself. This angered the father even further, so he took the knife and attacked the mother with it then came at the Joker and stuck the blade in his mouth, slicing a long gash of scars through each of his little cheeks and leaving him with a Chelsea Grin.
Following the incident, the Joker grew up as an orphan being humiliated and frightened by his friends and family due to the scars, which started to heal as time went by. His luck changed in the other story he related on, however, when he married a beautiful addicted gambler who always told him that he shouldn't worry too much and should smile more often. This advice went overhead when the wife fell into a difficult debt with enforcers of loan sharking who eventually carved her face as a retaliation for her lack of payments, and without enough money for her nor the Joker to afford cosmetic surgeries, she was miserably forced to endure her disfigurement. The Joker didn't even care about the damage to her appearance and just wanted to see her smile again, getting him stuck with belief that the advice to smile more often and permanent smile the scars on her face resembled corresponded to the same nightmarish memory of his father slicing the similar scars across his son's mouth, which had then came back to haunt him, this time forever. To show empathy or encourage a feeling of solidarity, the Joker remade his own scars into matching his wife's by stuck a razor in his mouth and cut his own face, but this instead got her to leave, unable to stand the sight of his new appearance and leaving the Joker always smiling.
No matter how much he missed, the Joker was driven into madness over the fact that she and the rest of his family and friends couldn't see the sense of humor behind his view to always smile like his scars do despite the horrid sight it makes because all the world cares about is the money to get rid of every atrocious sights they are unable to tolerate, which most people see often leads to corruption, and desires to earn that respect he wanted from the world by upsetting their social order through crime, believing that superficial, meaningless chaos and selfishness was basic human nature.
One month after Batman made presence known in Gotham City, the Joker began making his presence known by committing armed robbery and double homicide, leaving a joker playing card at the crime scenes (which he supposedly earned his name from, with another being that he wears makeup on his face to scare people). Over the coarse of eight months, he enacted in various heists with his gang while disguised as one of the accomplices. The last of these reported robberies was that of the Gotham National Bank, which a group of his men dressed as clowns (Bozo, Grumpy, Chuckles, Happy, Dopey and an unnamed bus driver) who he ordered to kill each other during the heist. Thus, the only thug left standing was Bozo, who was revealed as the Joker himself to the crippled bank manager before making off with the money in a school bus. Interrupting videoconference mafia leaders Sal Maroni, the Chechen, and Gambol had with the Chinese billionaire money-laundering accountant Mr. Lau, about the remaining laundered funds that he hid from the authorities while fleeing to Hong Kong, the Joker explained that Batman is unhindered by jurisdiction, and offered to get him out of the way by killing him in return for half of the money. While the other mafia bosses decline the offer for a while, Gambol refused it by putting out a bounty on the the Joker's head. Later, pretending to be dead, the Joker killed Gambol by sticking a knife in his mouth and telling the gangster 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 the story of his ex-wife until Batman appeared and attacked him. In response, the Joker held Rachel hostage and threw her out of the window, leaving Batman rushing rushing past to save her while the clown 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.