[4][5] Even as a child, Jack Napier was psychologically unstable, but extremely intelligent, showing an advanced knowledge of chemistry, art and science; he was also in and out of juvenile detention facilities for crimes such as arson, assault, and grand theft auto.

"[6] Years later, Napier moves up in the ranks of the Gotham City mafia, eventually becoming the right-hand-man of crime boss Carl Grissom (Jack Palance).

Grissom finds out about the affair and sets him up to be killed by Lt. Max Eckhardt (William Hootkins), a Gotham City Police Department cop on his payroll.

Gotham City Police Commissioner James Gordon (Pat Hingle) and the masked vigilante Batman (Michael Keaton) intervene separately, disrupting the theft.

He survives, but the chemicals turn his skin chalk-white, his lips red, and his hair green, while a botched attempt at plastic surgery leaves him with a permanent rictus grin that coincidentally makes him look like the Joker from his cards.

Styling himself as "The World’s First Fully-Functioning Homicidal Artist", Joker seeks revenge on Batman for his disfigurement and becomes obsessed with "outdoing" the Dark Knight, who he believes is stealing the spotlight from him.

Aided by his right-hand man Bob (Tracey Walter), Joker massacres the rest of Grissom's associates at a press conference and begins poisoning cosmetic products with "Smylex", a chemical agent that causes its victims to laugh hysterically as they die, leaving their corpses with a permanent smile.

Joker also becomes obsessed with photographer Vicki Vale (Kim Basinger), and attacks her while she meets with her boyfriend, billionaire Bruce Wayne – Batman's alter ego.

Joker keeps his promise of giving away the money, before releasing an airborne toxic version of Smylex onto the crowd via parade floats, killing dozens of people.

Batman arrives in the Batwing and removes the balloons before Joker uses a revolver to shoot the plane out of the sky, causing it to crash on the steps of the Gotham City Cathedral.

As they fight, each admits that he is responsible for the existence of the other after Joker remembers the alley murders and realizes that Batman is Wayne, frenziedly shocked of knowing that he is to blame for his own plight.

Burton initially wanted to cast character actor John Glover (who would later appear in Batman & Robin as Dr. Jason Woodrue, Mr. Sivana in Shazam!

Nicholson's contract featured an "off-the-clock" agreement, specifying the number of hours he could have off, and allowed him to take time off to attend Los Angeles Lakers home games.

[18][19] The character's origin in the film of falling into a vat of chemicals was inspired by the then-recent graphic novel Batman: The Killing Joke, written by Alan Moore.

The real Joker had appeared in the novel via flashbacks, including that he had encountered the meek Selina Kyle outside the Flugelheim Museum after Batman's escape with Vicki Vale and was a criminal acquaintance of her employer Max Shreck.

For his performance as the character, Nicholson was nominated for the Golden Globe for Best Actor in a Musical or Comedy, but lost to Morgan Freeman in Driving Miss Daisy.

[32] Nicholson was also nominated for the BAFTA Award for Best Actor in a Supporting Role by the British Academy of Film and Television Arts but lost to Ray McAnally in My Left Foot.

Throughout the music videos for the singles released from the album, Prince was dressed as an amalgam of Nicholson's Joker and Keaton's Batman in a persona he titled Gemini.