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 her, 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 mafia-owned 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 bosses Sal Maroni, the Chechen, and Gambol had with their 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 slicing his mouth with a knife the same way his father did to him, taking over his empire.
The next day, after learning that Batman has retrieved Lau, Maroni and the Chechen decided on the Joker's offer while recently-elected district attorney Harvey Dent prosecuted 549 of their men. Filming himself torturing Batman impersonator Brian Douglas in a slaughterhouse, the Joker announced to kill selected people each day unless Batman revealed his identity, stringing Douglas's body up outside Mayor Anthony Garcia's window and releasing the video on TV. Making good on this threat by first poisoning Gotham Police Commissioner Gillian Loeb and blowing up mafia trial Judge Janet Surrillo with a car bomb, he even targeted Dent at a fundraiser held in the DA's honor at the Wayne Enterprises penthouse, but Batman's billionaire identity Bruce Wayne hid Dent and left to suit up while his childhood friend and Dent's assistant district attorney Rachel Dawes was approached for Dent's whereabouts by the maniac, who she tried to drive away with a kick in the groin until Batman appeared to finish the attack. 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.
Following his murder of two men named Harvey and Dent, which he left a newspaper article with a picture of the mayor circled and labeled as his next threat, the Joker disguised himself and his men as the honor guards at Loeb's memorial service to assassinate him. In the resulting mass hysteria, the Joker and his men got away, leaving one of them, Thomas Schiff, behind when he is shot in the leg. Although Garcia survived, Police Lieutenant James Gordon was apparently killed instead after saving him by pushing him out of the way of the Joker's bullet. When the Joker named Rachel as his next victim through Schiff, Dent attempted to draw him out by falsely identifying himself as Batman, the Joker diverted the DA's police transportation using a burning fire engine, then ambushed the convoy in a semi-truck using a shotgun and bazooka. However, the real Batman showed up in The Tumbler, and saved Dent from the Joker's missiles. Now aware that Dent probably wasn't the Bat after all, the Joker took the wheel of his truck and faced Batman, who flipped the truck and crashed the Bat-Pod in resistance on killing him, leaving the clown's arrest open for Gordon, who had faked his own death and was rewarded the title of Police Commissioner immediately aftewards. Thus, the Joker was successfully jailed at the Major Crimes Unit, which he later turned out to have planned the whole time so that he could retrieve Lau, who was also held there, to gain control of the mafia's funds.
When Dent and Rachel were kidnapped by by the mafia through their inside agents in the Gotham Police Department and put in warehouses containing barrels of oil, the Joker was interrogated by Batman, whom he recognized along with himself as freaks outside society before revealing the two locations of both DAs. However, he turned out to have switched their whereabouts to trick Batman into saving Dent, who is disfigured in the explosion while Dawes is killed in the other. Then, talked Police Detective Gerard Stephens into attacking him and holding hostage with a piece of broken glass, he escaped captivity with the help of a pre-planted phone-activated bomb sewn into one of the henchmen who was arrested with him, Kilson, and Lau in tow. Meeting with the Chechen at the location of the hidden funds, the Joker killed him and Lau by burning the latter alive with the money and ordering the former to be fed to his rottweilers, taking over their empires. Then, to prevent Wayne Enterprises M&A law accountant Coleman Reese from going public on TV with information on Batman's true identity, he threatened on the phone line to bomb a hospital unless Reese is with in an hour, causing civil disorder against the accountant and the evacuation of all local hospitals. During the panic, the Joker snuck into Gotham General Hospital dressed as a nurse to meet with Dent, who raged over Rachel's seath, and convince to seek revenge by embracing anarchy, then blew up the hospital during their departures and escaped with a bus-load of hostages.
As Gordon called for the National Guard to secure the city, the psychopath announced his regime to Gotham through one of the hostages, reporter Mike Engel, and warned the citizens to flee if they don't want to cooperate, placing them in an emergency evacuation by ferries. To escalate the chaos, the Joker had two of the ferries separately containing civilians and convicted criminals rigged with explosives, and gave the passengers of each ferry decision to blow up the other by midnight otherwise he'll blow up both from the Prewitt Building construction site. Batman intervened after saving the hostages (who the Joker dressed as his henchmen to lure Gordon's SWAT team into targeting them) and disarmed the Joker of the detonator to both ferries, capturing him with a rope. Although neither ferry exploded, the Joker revealed how his plan was really gonna work: by driving Dent insane to the extent of capturing Gordon's family so that the city would lose hope once the scarred DAs rampage becomes known. Horrified, Batman left the Joker's arrest to the SWAT team, but not before the clown reminded him that there fight may last forever.