Origins unknown, it is commonly believed a lab assistant at Ace Chemicals moonlighting as a failed stand up comedian was pressured to join the Falcone mob. He wore a suit and tie with the Red Hood. After learning his wife and unborn child were killed in an electrical shortage, he went on a string of robberies. In a confrontation with Batman, he opted to jump into a vat of chemicals rather than face arrest. He was sucked down a drainage pipe and discovered his skin was bleached white, hair dyed green and face forced into a permanent smile. Driven over the edge, the man was reborn as the Joker, a homicidal maniac with a lust to create social disorder and anarchy. With a capacity of incredible violence, a brilliant mind to concoct elaborate schemes and a morbid sense of humor, the Joker is one of Gotham's most dangerous super criminals.
At some point, the Joker threatened the city with a dirty bomb. It reportedly had enough payload that the radiation could kill half the city's population. While Batman searched the city, the Joker was incarcerated at Arkham Asylum. During a Task Force X mission, the Joker sighted his recent ex-girlfriend and ally, Harley Quinn. After he taunted her, Harley stole a handgun off the disguised Deadshot and opened fire on Joker's cell. After she got up close, she aimed inside and fired. While he hid from the richochet, Harley fired repeatedly on a part of his cell's wall. Harley in turn taunted him and introduced her new boyfriend. Joker recognized Deadshot, whom he hired once to kill Batman while he was disguised as the mob boss Black Mask. Eventually, the Joker noticed the shot up panel and crossed two wires to open his cell. Now free, the Joker gathered his clothing and killed two guards during a blackout. He later came upon a wounded Batman. Just when he was about to shoot Batman, Joker watched as Batman's head was blown off.
Upon further inspection, Joker determined it was Black Spider and listened in on his comm. Hearing Harley's screams, he rushed over to the medical center and attacked Task Force X, Batman, and the Riddler alike. After Joker shot Batman as he swung to safety, he cut off Task Force X's path. Temporarily content with renewing the past with Harley, Joker confronted Deadshot. Surprisingly, Deadshot called his bluff and walked up to Joker's handgun then started a countdown. Unexperienced with someone lacking the will to live, the Joker examined the gun and was bashed by Deadshot. Joker took Harley and dropped down a nearby laundry chute. He took Harley's mallet and activated his dirty bomb, hidden in it all along. The Joker hijacked the feeds in Arkham and announced his plans to detonate the dirty bomb then released all inmates at Arkham Asylum. During the chaos, Joker and Harley stowed away on a police helicopter and didn't reveal themselves until Deadshot piloted it towards the city. Deadshot and Joker grappled as Harley attempted to keep the helicopter in the air.
Batman soon intervened in the battle. After the helicopter crashed into a building, Joker continued his battle and used a chain and knives on Deadshot. Deadshot gained the upper hand and threw him back in the helicopter. Before he could get up, Joker was pinned by his own knives. Deadshot jumped from the nose of the helicopter, forcing it to fall several stories. The Joker simply laughed at his predicament and escaped certain death while Batman defused the dirty bomb with one second left.