The story of the evil Clown Prince of Crime known as The Joker from the DC Animated Universe.
The novelization of Batman: Mask of the Phantasm indicates that the man who went to become the Joker was born around 1948, being 35 around the time of the flashback events of the movie, whose present time is 1993. He became a criminal at some point to make a living, a profession which would last for the rest of his life. It's unknown what the Joker's real name was, and while it's suggested to have been Jack Napier, it's possible that this may have been another alias, as he took several during his career. The reason was possibly to avoid getting detected by the police.
By 1983, he was an assassin and chauffeur working for Salvatore Valestra, Buzz Bronski and Chuckie Sol, the high-ranking members of the Valestra mob. When Carl Beaumont, a businessman who had embezzled from the mob, was unable to pay them back, he accompanied his boss and associates to pay a visit to Beaumont, who asked Valestra for one more day. Valestra reluctantly agreed, but unknown to him, Carl and his daughter Andrea fled the country that same night.
Beaumont paid Valestra back, but the mobster wanted "interest compounded in blood" and sent the hitman to Europe to find and kill Beaumont. Arthur Reeves, Beaumont's former employee, gave them Beaumont's location in exchange for funding his campaign for Gotham City Council. The hitman killed Beaumont, and passed by Andrea on his way out, greeting her nonchalantly moments before she found her father's body.
Eventually, the hitman formed his own gang. One night, he robbed the Ace Chemical plant, but Batman arrived, and in the ensuing scuffle, the hitman fell into a vat of chemicals. The chemicals beached his skin white, turned his hair green, made his lips bright red; driven insane by his reflection, the hitman reinvented himself as the Joker and dedicated his life to fighting Batman. After taking up his new identity, Joker would establish himself as one of the main crime bosses in Gotham and became Batman's greatest foe. During a stint in Arkham Asylum, the Joker manipulated his psychologist, Dr. Harleen Quinzel, into falling in love with him, driving her to join his gang as Harley Quinn, his accomplice and on again-off again girlfriend.
Batman: The Animated Series
As the Joker, he pulled numerous schemes for both his own amusement and for personal profit, such as a Christmas-themed kidnapping of Commissioner Gordon, Summer Gleeson and Harvey Bullock ("Christmas with The Joker"), filling Gotham City with his laughing gas using a garbage boat ("The Last Laugh"), planting dynamite at the birthday party of Mayor Hill's son ("Be A Clown"), poisoning all of Gotham's fish with his toxins and trying to copyright them ("The Laughing Fish") and escaping Arkham to blow up a casino that was using his name sake ("The Joker's Wild"), but Batman and Robin always managed to defeat him and send him back to Arkham.
In "Joker's Favor", Joker tried to blackmail a man named Charlie Collins after he encountered him on the highway, and threatened to kill his family if he didn't help him in his scheme to kill Commissioner Gordon during an award ceremony. Collins ended up getting the better of the cowardly psychopath, using one of his own fake explosives to trick him into giving up everything he knew about his family to Batman.
In "Almost Got 'Im", Joker, Penguin, Two-Face, Killer Croc, and Poison Ivy get together and play poker, remarking on the times in which they had nearly killed Batman. Joker, believing his story was the best, saved it for last, mocking the stories of his fellow criminals. When it was his turn, Joker pulls out a portable television set and played a recording of his failed attempt. In the tape, Joker and Harley Quinn had taken over Late Night Gotham Live, captured Batman and tried to electrocute him on live TV with an electric chair powered by the forced laughter of the kidnapped studio audience, but this attempt was foiled by Catwoman. Due to only being foiled by the appearance of Catwoman, all the other villains agree that Joker had the best story, while he simultaneously wins the poker game (by cheating). Joker then reveals that he had kidnapped Catwoman and was planning to kill her by turning her into cat food as revenge for her ruining his plan, although, he is then immediately knocked out by Batman (who was disguised as Croc the entire time) .
In the movie Batman: Mask of the Phantasm, a vigilante called The Phantasm came to Gotham and, one by one, killed the mob leaders Joker had once worked for. Batman was accused of these crimes, but the Joker doubted this and set up a trap to kill the Phantasm before he himself was targeted. Joker eventually managed to find out that the killer wasn't Batman at all and that Andrea Beaumont, daughter of the embezzler he once killed on mob orders, had taken up the identity of the Phantasm. He fought the Phantasm, but she vanished to escape in an explosion, taking the laughing Joker with her. It remained unknown what happened to him after this. But he survived off-screen and went back to do what he does best — fighting Batman and terrorizing Gotham City with his deadly jokes.
In Trial, along with other inmates in Arkham Asylum, Joker ran a sham trial for Batman with the inmates as judge and jury.
The Joker later gotten a hold of an atomic bomb, taken the mayor hostage (although this was one of the very few times that he showed affection towards Harley when he realized that she had enough ‘guts’ to kill him), and even tried to succeed as a stand-up comedian while ruining three comedians’ acts for revenge on their judgment of him as unfunny — something he could never forgive. After this, he tried to destroy all Gotham City with his henchmen, but Batman, Robin and Harley Quinn destroyed his plane and defeated him.
Superman and Financial Failure
Becoming broke and desperate, Joker and Harley Quinn stole a statue of a Chinese Dragon that turned out to be forged from Kryptonite. Using this, Joker traveled to Metropolis and put himself for hire for one billion dollars to kill Superman. Lex Luthor agreed to Joker's deal. Batman eventually fought The Joker along with the Man of Steel. Then, Joker began to annoy Luthor when he demanded to be paid more to kill both superheroes. While attempting to betray Luthor (who needless to say, had also tried to betray him) The Joker ended up fighting the World's Finest in an aircraft, the explosion of which seemed to kill him, though his body was never recovered. But he survived that fall and went back to terrorize Gotham again off-screen.
He turned up alive, was captured, but was shocked when he inherited millions of dollars from a mob kingpin named Barlowe, who had always despised him. Through deception was incredibly and playing the Joker's ego and greed, Barlowe set up a trap. Joker almost literally bought his way to a clean criminal record and then lived the high life on what he believed to be hundreds of millions. It was eventually revealed that only the first ten million dollars were real, which the Joker not only blew through but was now, as Barlowe's heir, in the sights of the IRS. Joker could not admit the truth, or else he'll look like the clown he was to the criminal underworld. Joker decided to pull off a heist to restart his enterprises (without using any of his calling cards or signature styles that would link him to the crime) but was foiled, like always, by Batman. As he is taken back to Arkham Asylum, Harley Quinn (disguised as a police officer) beats Joker up with a baton for abandoning her to be arrested by Batman and Batgirl in the beginning of the episode and replacing her with a Fake Harley, rather than bailing her out.
On the seventh year anniversary of the Joker's "birth", the Joker's actions turned reporter Jack Ryder into the Creeper. The Creeper, while heroic, mercilessly harassed Harley and Joker, to the point that even Joker considered Creeper to be a lunatic, actually begging Batman to arrest him. Creeper captured Harley and defeated the Joker in a dump park, who was later taken in custody by Batman.
After this, Harley Quinn captured Batman and tried to kill him to obtain Joker's admiration, but he hit her and threw her through the window of the building, believing that no one had the right to kill Batman but himself. Batman eventually frees himself and fights Joker on top of a monorail, and punched the Joker into the chimney of Ace Chemicals, apparently killing him. But for unknown reasons, he survived the mortal fall and sent some flowers and a note to Quinn to get her back on his side.
Later life and Justice League
The Joker later planned on forming a metahuman gang from the local mutants known as "Bang Babies". Villains such as Hotstreak and Talon eagerly joined his gang. However, Batman, Robin, and Dakota's own resident hero Static teamed up and escaped the Joker's death trap, arresting the Clown Prince of Crime in the process. Joker tried to shock Batman with a joy-buzzer after his defeat, only for Static to not only ignore and absorb the buzzer's power but shock the Joker right back, only worse. Joker and his gang were arrested after that.
Not wanting anything to do with Joker after their first encounter, Lex Luthor tried to keep him out of his Injustice Gang, but he is ultimately persuaded to relent by him after Copperhead is arrested and Joker provides knowledge on Batman. Using a tracking device Batman had planted on him, Joker lured Batman to the Injustice Gang's hideout and knocks him out. With Batman now captured, Joker urges Lex to let him kill Batman but is frustrated when he is not allowed to and then proceeds to annoy Lex as he searches through Batman's utility belt. Joker then forces Batman to watch the expected destruction of the JLA's Watchtower, knowing that Luthor would give him permission to kill Batman if his plan succeeded, but it fails and Joker is not allowed to finish Batman yet again. Knowing Batman was swaying her to his way of thinking, Joker betrays and electrocutes Cheetah into unconsciousness. During the Gang's final confrontation with the JLA, Joker uses a trick exploding doll and several marbles to attack Flash and Wonder Woman, but quickly runs away when he realizes that he's outmatched. After the rest of the Gang is defeated, Joker rushes to the basement of their warehouse base to kill Batman, only to be knocked out by Batman, who states that he could have escaped their prison at any time he wanted, but chose to stay and watch on them instead.
He discovered and entered a secret government compound dubbed Section 12. At the time, it was under the supervision of the Cadmus Project, a federal initiative to procure defenses against the Justice League in case they ever went rogue. Joker arrived at the facility and killed the staff using his Joker Venom. He then freed their captives and fashioned them as a card deckhand, dubbing them his 'Royal Flush Gang'. During his time at the facility, the Joker also came across some of Project Cadmus' secret technology and discovered a microchip that allowed it's users to copy their DNA.
Rather than use the technology early, the Joker decided to save the technology for another time. The Joker with his new team then left the facility before purchasing air time on several networks under the moniker of "Gwynplaine Entertainment" and broadcast a live feed of Las Vegas. The Joker secretly planted several dozen extremely powerful time-bombs all throughout the city and threatened to blow up the entire city early if anyone but the Justice League tried to stop him. Though the Justice League manage to disarm the bombs and defeat the Royal Flush Gang, the bomb stunt was merely to attract viewers throughout the world. The real plan was to use the powers of the Gang's fifth member, Ace, who could drive people insane just by looking at them in person or on TV. The Joker then transmitted Ace's thought waves across the air to render everyone under a mass psychosis. Batman confronted the Joker alone, and despite almost being driven insane by Ace's powers and receiving vicious beatings from Joker, managed to reveal to Ace that the Joker held on to a special headband used to nullify Ace's powers by Project Cadmus. Ace, in anger, used her power on the Joker and temporarily incapacitated him, rendering him temporarily catatonic.
Years later, he planned one final joke on Batman after realizing the "game" was getting old. The Joker and Harley Quinn kidnapped Tim Drake (the current Robin and Dick Grayson's successor) after Tim saved an unknown woman from a criminal, unaware that it is a trap and subjected him to three weeks of torture and brainwashed him into becoming a small version of the Joker called Joker Junior, much to Batman and Batgirl's horror, also finding out all of Batman's secrets (including Batman's true identity) in the process. On top of that, he documented the whole interrogation with an old video projector and possibly slides (since he claimed he had them). Batman came to the abandoned Arkham Asylum and saw what had happened to Tim. In retaliation, Batman engaged in a bloody fight with the Joker (willing to break the Joker in two for his actions), only for the Joker to stab Batman in the knee and gloat over his success (in the edited version, he punches Batman off the edge and the blood was removed).
He gave a Bang-Flag gun to Tim to finish off the Dark Knight, but instead Tim focused his rage on the one who had broken him, and fatally shot the Joker, (in the edited version, he pushed Joker into liquids (possibly water), who slipped and grabbed the lever and pulled it down as he was slipping, electrocuting himself) and says to himself, "That's not funny! That's not..." before he dies. Tim then enters into a mental breakdown, crying whatever sanity he has left of himself as Barbara Gordon (the current Batgirl) comforts him.
The remains of the Joker were buried under Arkham, and Tim was given therapy to cope with his trauma. Commissioner Gordon learned of what happened and promised to keep it a secret, while it seemed Harley had fallen into the bottomless pit and perished during the battle as well. The other people who also knew the incident were the Dark Knight's butler, Alfred Pennyworth, and Robin, who has left for Bludhaven. One year later, Tim was finally recovered from his trauma, but Bruce forbade Tim from being Robin again and swore that he will never endanger another young partner again. Tim soon left after this, choosing to make the right decision for himself. Barbara soon retired from her career as Batgirl and went on to follow her father's footsteps as the new police commissioner of Gotham City following her father's retirement. Despite his death, Joker's legacy as a criminal mastermind and homicidal maniac would live on for years. Whilst the people of Gotham were relieved at the Joker's final and ultimate defeat, they still feared his destructive legacy. Years later several groups of criminals called the Jokerz started appearing around the globe, honoring the Clown Prince of Crime's legacy. One of the gang's members, Ghoul (who coincidentally would be the member of the Jokerz gang that would serve the newly revived Joker.) stated that there are 9,453 active Jokerz that are divided into approximately 200 separate groups. Bruce continued the rest of his crime-fighting career as Batman for years after the incident a few years before his retirement in the prologue of Batman Beyond.
Legacy (Return of the Joker)
It seemed not even death itself could restrain the Joker. It turns out that before Joker died, he had implanted the microchip he stole from Cadmus encoded with his DNA, memories, and personality on Tim's neck. The chip soon began to activate itself automatically, allowing the Joker, who was the original Joker's son, to transform Drake into him, though he wasn't strong enough to permanently transform. According to the Joker, Drake chalked up any lingering memories as bad dreams.
Over the course of forty years, the Joker, who planned to avenge his late father, plotted his father's mantel return using Drake's body. With Drake's knowledge of communications and his martial skills as Robin, Joker staged a series of corporate thefts utilizing a sub-group of The Jokerz which had formed in his absence, proving how influential he had become. With the technology, Joker created a satellite jammer and hijacked control of a Hyperion class defense satellite orbiting the planet. If any of Tim Drake's family were worried in any way, Joker would simply call on the phone with Tim's voice saying he'll be working late.
Joker then revealed himself to Bruce Wayne at a party during his return to Gotham. The new Batman managed to stop Joker from killing Bruce, but Joker escaped. After learning what happened to Tim, Terry deduced Drake was involved and decided to confront The Joker.
However before he could reach Joker's hideout, Joker tested the new Batman by chasing after him using his satellite to send a powerful laser at the Batmobile whilst incinerating a small part of Gotham. However, the Joker abruptly stopped the beam shortly after, allowing the new Batman to track Joker down to his hideout (an abandoned Candy Factory called the Jolly Jack) and confronted Tim. Tim started to remember how he killed the original Joker and started to act strangely; when Tim accidentally mentioned the new Batman's real name, he transformed into the Joker, revealing he had deduced Batman's identity. After explaining how he survived, Joker and Batman fought.
In the skirmish, the electric joy buzzer that Joker was using gets knocked out of his hand and falls into the wiring of the jamming system, redirecting the satellite's laser straight to Joker's hideout. Batman stranded Joker within the compound and fought him one on one. In the finale, Batman taunted the Joker, saying he had only returned because he could never make the original Batman laugh. This greatly enraged Joker and he managed to trap Terry under a wooden table and started strangling him.
Joker then ordered Terry to laugh before he dies and when Joker got closer in order to hear Terry better, Terry taunts by saying "Ha ha!" and uses Joker's own toy buzzer to electrocute him on the neck, much to his horror. The surge destroyed the microchip as the Joker let out a final scream, restoring Tim back to normal and destroying the Joker once and for all. After so many years of waging a one-on-one war, Batman had finally won after a long struggle, destroying the Joker forever.