The episode opens at the Ace Chemical Plant, where TV Reporter Jack Ryder is on-scene to do a special report on the Joker; today marks the seventh anniversary of the Joker's creation.
He explains that the Joker began life as an unknown criminal, who used a variety of aliases and began his career as a hitman for the Valestro mob. However, he eventually broke off and formed a gang of his own. On their first "job"; a raid on the Ace Chemical Plant, they encountered Batman. During the fighting, the man who became the Joker was accidentally knocked into a vat of chemicals, emerging as the disfigured sociopath he is now.
However, unbeknownst to Ryder, he has a special guest star; the Joker, Harley Quinn and a trio of bruisers are on the scene, intending to lure Batman for an "anniversary party". Just to ensure that Batman and Robin -who are watching the special at Wayne Manor and can see the Joker in the camera's picture- show up, Joker launches an attack against Ryder and his crew.
He bombards them with exploding rubber chickens, filled with his trademark Smilex -or laughing gas, as it is referred to in this episode-, and then hurls Ryder into a vat filled with the same chemicals that turned the Joker into his present self. Batman and Robin show up, and Joker directs Harley and the goons against them while he toys with Ryder some more, repeatedly pushing him back in before giving him an explosive cigar.
The flames from the cigar ignite the chemicals, causing explosions and choking smoke. Joker then attacks Robin and knocks him into a control mechanism, dumping the contents of the vat Ryder is in outwards. Asking Batman and Robin whether they're going to catch him or try and save Ryder, the Joker and his underlings escape.
Batman and Robin drive to the plant's chemical outlet, where all they find of Ryder is an empty jacket that dissolves into nothing. As the Batmobile drives off, a manhole it was parked over flies into the air as a strange, rag-clad figure leaps out of the hole. Jack Ryder has survived... in a manner of speaking.
His skin now died lemon-yellow, and his hair the same blackish-green as the Joker's, the grinning maniac giggles and gibbers to himself as he heads back into the city. Remarking that he's "feeling pretty good... for a dead guy!", Ryder darts through the streets and alleys, effortlessly leaping over several cars.
He darts past a reflective window, only to backtrack and stand with his back to it, disturbed. He slowly turns to see his new reflection and, in shock, leaps backwards- easily jumping the length of the street and catching himself on a lamp-post as he sails past it. The chemicals have obviously had a rather extreme physical effect on him.
Ryder tries to think logically and work out what's happened to him, but he's unable to concentrate for more than a few seconds before he experiences a sudden stab of pain. Remarking that he'll come back to that, he leaps up to an apartment building, peering in through the bathroom window and asking the occupant -a woman preparing for bed- for assistance.
She screams in terror and flees the room as Ryder quickly bounds to the street. Declaring that his career is ruined, Ryder swears vengeance upon the Joker. But his first matter of importance is to get some new clothing.
He leaps into a nearby clothing store, sending the customers fleeing in terror, with the exception of the check-out girl, who remains apathetic. Ryder digs randomly through the clothing and, finding nothing he likes, asks the clerk for advice. She offers a horizontally-striped black and green jockstrap, which Ryder eagerly grabs, leaping over into a changing room and grabbing a nearby mannequin modelling a large crimson boa, gloves and boots. Wrecking the mannequin in the process, Ryder adorns his new costume and bounds off into the night.
In the Batmobile, Batman and Robin receive a call from Alfred, who tells them that the police just received a report from a clothing shop that was trashed by a "giggling, lemon-yellow maniac", who paid for his purchases with Jack Ryder's credit card. Batman instantly turns the Batmobile in search of this stranger.
Joker heads towards his latest hideout, remarking that rumors are going around that someone is stealing his 'routine'. He enters the strangely dark hideout and calls for his goons, only to be told be Harley that she gave them the night off. The lights suddenly go on to reveal a giant banana cream pie, from which Harley emerges with a song routine, intending to celebrate her "puddin's" seventh anniversary of creation.
Unfortunately for her, the Joker instead throws her out of the hideout, telling her that if she wants to make him happy she'll find who's been stealing his act. As the despondent Harley leaves, the Joker tastes the pie's filling, gagging as he remarks that Harley's "a sweet kid, but a lousy cook".
Ryder leaps across the rooftops of the Gotham docks, heading for a seedy bar where the Joker's three goons are spending their free time. Ryder bursts in through the skylight, dropping onto their pool table. Ryder and the head goon engage in some banter, which results in Ryder taking a new name; "The Creeper", and then a brawl between the Creeper and the goons.
The sorrowful Harley approaches the bar just as the Creeper throws a pool table out through the window. As she looks in, the Creeper effortlessly defeats his opponents, utilising the inhuman strength and agility bestowed by his chemical bath to brutal effect. As the Creeper demands that the goons hand over the Joker, Harley asks what the Creeper wants with the Joker.
Unfortunately for her, the Creeper is instantly smitten the moment he lays eyes on Harley, attempting to chat her up whilst unconsciously knocking away the goons who attack him. He even sends Batman flying with a backwards uppercut. As the sole goon retaining consciousness suddenly starts shooting wildly at Batman and Robin with a machinegun, Harley flees the scene, the Creeper in hot pursuit.
Batman and Robin knock the goon unconscious with a pool table, then defeat the other goons, who have regained consciousness. Meanwhile, the Creeper surprises Harley further along the docks, effortlessly dodging her attacks and continuing his attempts to flirt with her. Harley eventually manages to escape him by dumping a massive crate of coal on his head.
As she runs off, Batman and Robin approach the ruined crate, which falls apart as the Creeper bursts out of the coal and, after some light banter with Batman and Robin, heads off in pursuit of Harley. Batman and Robin follow him discreetly as he heads through some alleys on all fours, sniffing the ground and poking his head into garbage cans like a dog.
A quote of his -"jilted madman stalks crazy clowngirl! Film at eleven!"- confirms Batman's suspicion's that the Creeper is Jack Ryder. He responds to Robin's disbelieving enquiry that the transformation must have been due to an interaction between the chemicals he was dipped in and the Joker's Smilex.
Harley bursts in on the Joker, almost causing him to drop the vial of nitroglycerine he's holding, and begs him to save her. As the Joker asks precisely who from, the Creeper tears through the wall. An argument between the Joker and the Creeper results in the Joker trying to blow the Creeper to Metropolis with his nitro, though all the explosion does is attract the attention of Batman and Robin.
Harley manages to knock the Creeper unconscious with a lucky mallet strike. Unfortunately, that's when Batman and Robin show up. Harley and the Joker manage to escape on a "It's a Small World" float, but the Creeper follows them in the Joker's car, bringing Batman and Robin along for the ride. A wild chase follows, ending with the Creeper holding Harley hostage atop a mountain of garbage, the Joker willingly surrendering to Batman to escape the insane -even by his standards- Creeper.
Batman knocks the Creeper out with a drug injector from his utility belt, and the scene changes to Jack Ryder's apartment, where Ryder, Creeper no more, sits in just the Creeper's jockstrap, a strange patch on his arm. Batman explains that the patch is a temporary antidote, and that he'll need to make more for Ryder.
He tells Ryder that his career is salvageable, as no one knows that Ryder and the Creeper are the same person. Batman and Robin swing off into the night as Ryder stares at the patch. He then begins to slowly peel it off. The scene changes to an outside view of Ryder's window as the Creeper's laughter begins to resound.