At the Ace Chemical Plant, news reporter Jack Ryder is anchoring a live TV special on the career of the Joker, starting with his encounter with Batman at the plant that ended with his life-changing tumble into a vat of chemicals.
Joker, watching from above, decides to stage his own tribute to that fateful day, entering onto the scene, dosing Ryder and his crew with his trademark laughing gas, and then "re-enacting" his confrontation with Batman by tipping Ryder into another large vat of chemicals. Batman, having glimpsed Joker on the television, is quick to arrive at the plant with Robin. While Joker's henchmen keep them busy, Joker occupies himself with pushing the hapless Ryder back into the chemical soup every time he tries to climb out. For his finale, Joker gives Ryder an exploding cigar causing the chemicals to explode. Joker then fights Robin knocking him into the plant's controls causing the tank to flush into the nearby sewer, forcing Batman to try and save Ryder, letting Joker escape.

Ryder's burning coat is seen dissolving into the chemicals and he is presumed dead, but in fact Ryder has survived, and had a bizarre reaction to the combination of Joker gas and the chemicals: he is transformed into an extraordinarily strong and agile maniac with yellow skin, green hair, and a rictus grin. Ryder runs around town for a while and decides that he should get revenge on the Joker. Ducking into a thrift store, he assembles a bizarre costume, preparing for a new career as a crime-fighter. He uses his own credit card, alerting Alfred to the transaction. Alfred then alerts Batman to the Creeper's location.
Joker returns to his hideout upset that someone is stealing his image. However, he finds that his henchmen are gone, Harley has given them the night off, so she can throw a special, private party for Joker's birthday. She rises out of a giant pie and offers to play but Joker predictably, tosses Harley out on her ear telling her to "find the plagiarist who's been stealing my act!"
Ryder's finds Joker's thugs at the Stacked Deck, he demands Joker's whereabouts, dubbing himself (based on one of Mo's remarks), "The Creeper!" The three thugs attack Ryder but he has no trouble taking them down. Harley starts moping outside the club when Ryder catches site of her. Feeling an intense attraction, he accosts her, demanding "the inside story on the Joker... and your phone number."

Harley runs from the club and Ryder is instantly after her. She tries to kill him by dropping a heavy crate onto him but Ryder is unharmed. Pursuing Harley back to Joker's hideout, the Creeper confronts the Joker who throws a vial of explosives at him. However, the Creeper is still unharmed and is only defeated when Harley hits him with her hammer into a stack of giant blocks. The explosion leads Batman and Robin there, where they begin their pursuit of Joker and Harley. The Creeper wakes up and pursues Joker and Harley as well. However, being in the Jokermobile he accidentally unleashes missiles behind him. He then attacks Joker. Although his mania is benign, his methods are so extremely wild and frantic that even the Joker begs to be arrested, crying "He's a lunatic!" Batman cuffs Joker and injects the Creeper with a sedative.
Back at Ryder's apartment, Batman applies a skin patch that returns Ryder to his normal state. Before leaving him to rest, Batman warns him that the patch must be left on to counteract the chemicals. After they leave, Ryder examines the skin patch curiously, finding it hard to believe that a "little piece of cotton" can make such a big difference. Outside his apartment, there is a small sound of tearing fabric, and soon the Creeper's demented laughter is pealing from the window.