Smylex was the Joker's codename for any of his poisoned products that, when paired in specific combination, caused their victims to begin laughing uncontrollably, and then fall down, with their facial muscles pulled taut in a rictus grin. It could also be processed in a gaseous state.
Mayor Borg of Gotham City ordered District Attorney Harvey Dent and GCPD Commissioner James Gordon to increase police activity and combat crime in preparation for the city’s bicentennial. Reporter Alexander Knox and photojournalist Vicki Vale began to investigate reports of a vigilante nicknamed "Batman", who targeted the city's criminals.
Gotham mob boss Carl Grissom, who was already targeted by Dent, discovered that his mistress, Alicia Hunt, was involved with his second-in-command, Jack Napier. With the help of corrupt policeman Lt. Max Eckhardt, Grissom set up Napier to be murdered during a raid at the Axis Chemicals. During the ensuing shootout, Napier killed Eckhardt, after which Batman suddenly appeared. The two struggled, and Napier was accidentally knocked into a vat of chemicals. Batman fled, and Napier was presumed dead.
In actuality, Batman was Bruce Wayne, a billionaire industrialist who, as a child, witnessed his parents’ murder at the hands of a mugger. Bruce met and fell for Vicki at a fundraiser at Wayne Manor, and the two began a relationship. Meanwhile, Napier survived the accident, but was horribly disfigured with chalk-white skin, emerald-green hair and a permanent ruby-red grin. Driven insane by his reflection, Napier reinvented himself as "The Joker", who then killed Grissom and usurped his criminal empire.
The Joker began to spread terror in the city, first by lacing hygiene products with a deadly chemical known as "Smilex", which caused victims to laugh to death when it was used in certain combinations. The Joker then set a trap at the Flugelheim Museum for Vicki, and he and his Goons vandalized the surrounding works of art. Batman arrived, rescued Vicki, and the pair escaped in the Batmobile. Batman gave information about Smilex to Vicki so that she could warn the city via the Gotham newspapers about the poisoned products.
Bruce met with Vicki at her apartment, and was prepared to tell her that he was Batman. They were interrupted by the Joker and his Goons, who then asked Bruce: "Have you ever danced with the Devil by the pale moonlight?" before he shot him. Bruce, who wore body armor, escaped, recollected that the man who had killed his parents asked him the same question; then he realized that the Joker had killed his parents. Vicki suddenly appeared in the Batcave, and was let in by Bruce's butler, Alfred Pennyworth. After he avouched himself to Vicki, Bruce — as Batman — left to destroy Axis Chemicals. Meanwhile, the Joker lured Gotham's citizens to Gotham's 200th Anniversary Parade with a promise to give away $20 million in cash. When the citizens arrived, however, Joker attacked them with Smilex that spewing fromthe inside of his giant Parade Balloons. Batman arrived on the scene and saved Gotham from the attack by using the Batwing, but Joker kidnapped Vicki and took her to the top of the Gotham Cathedral.
Batman pursued the two, and at the top of the Bell Tower, he and Joker confronted each other in single combat. When the Joker attempted an escape via Helicopter, Batman grappled his leg to a heavy stone gargoyle, which caused him to fall to his death. Commissioner Gordon unveiled the Bat-Signal, along with a note from Batman that was read by Dent, which promised to defend Gotham whenever crime struck again.
- The gas was based on the comics' "Joker Gas", and any other alias for the poison that left the Joker's victims with a postmortem rictus grin.
- In Sam Hamm's original script, the first time that we saw Smylex's effect (called Smylenol in the script) was on the two female models, who were only represented as cardboard cut-outs in Joker's commercial. The original scene had them in a bikini photo session with a photographer. The photographer urged them to smile more as he snapped away. The girls began to giggle, which at first pleased the photographer. Soon, their giggles became laughter before they became uncontrollable helpless hysterics, which had the photographer going from mild annoyance to complete horror as the exhausted girls expired from forced hilarity, with the ghastly Joker-like grins that were frozen on their faces.
- As it was originally intended, the death scene was much more protracted than the one with the female newscaster, which depicted death by Smylex as a particularly agonizing, if mirthful, way to go.
- In the novelization of the film, the photographer was revealed to be Vicki Vale.