Mayor Hill is giving a speech at a construction site, saying the new block of apartments being built will herald a new, safer Gotham City. The speech is rudely interrupted by a pair of criminals who crash their getaway car and then open fire on the police pursuing them. Batman appears and takes them down. Rising from the podium he ducked under, Hill resumes the thread of his speech, railing against men who act outside the law. When Summer Gleeson sardonically asks him if he includes Batman, Hill responds, absolutely: "He and criminals like the Joker are cut from the same cloth." Joker happens to be watching this in his hideout, and is outraged at the comparison. He gets an idea for revenge from Hill's next statement, promising to make the city "as safe as [his] own mansion."
That weekend, the mansion is the site of a birthday party for the mayor's son, Jordan. The party is more a political media event than a real celebration, and Jordan feels overlooked and unloved. Practicing magic tricks alone in his room, he is mad when his father barges in and demands that he come out and greet the cream of the city. Hill has a surprise for his son, though. In addition to the important guests, he's invited a clown: the exuberant Jekko the Magnificent. Jordan is awed by Jekko's act, and asks how he can become a magician like him. Jekko laughs and says the first necessary step is to run away. Then Mayor Hill insist that Jordan leave the clown and come and greet the new arrivals, including Bruce Wayne. Fed up, Jordan runs up to his room.
For his parting gift, Jekko plants a large sparkler candle on top of the birthday cake It doesn't take a genius to figure out that Jekko is really the Joker, and the candle is really a stick of dynamite, but no ones ever accused the average Gothamite of being brilliant. As he leaves the Mayor, he lets out one of his trademark laughs. Hearing it, Bruce looks around and sees the candle. Pushing his way through the crowd, he pretends to stumble with the large present he's carrying, knocking the cake into the pool, where the bomb detonates harmlessly underwater. Hill calls the police, who find the real Jekko the Clown tied up down the road from the mansion. As Hill demands answers, Bruce realizes, "where's Jordan?" The scene shifts to inside Joker's van, where Jordan pokes his head up. Joker returns to his hideout, an abandoned amusement park. He starts to remove his makeup, but Jordan appears, saying he followed "Jekko's" advice and wants to be a museum like him. Joker is initially enraged at having his hideout invaded, but laughs it off and says he's been thinking about a protégé. Reviewing a home video of the birthday party, Bruce notices "Jekko" mention the name of his mentor, the "Great Prosciutto" – who was a performer at the amusement park. Back at the park, Joker is teaching Jordan how to swallow a sword. His lesson is interrupted when Joker's security system sounds an alert. As he scans his cameras, he notices Batman skulking through the park. He proposes that he and Jordan play a little joke on the Dark Knight and Jordan, remembering that his father says Batman is no good, and not wanting to upset his new mentor, agrees. Jordan lures Batman into the park's funhouse, where Joker ambushes him with his razor-tipped playing cards. Batman easily evades the attack and chases after Joker, who flees to higher ground and then tosses his ace card at Batman. This card detonates into a cloud of knockout gas, and Batman falls to the ground. Joker uses a fortune telling machine which drops a card predicting Batman's fate. Joker reads it and laughs, "You're gonna love this!" When Batman wakens, he is upside down in a tank rapidly filling with water, bound by a straightjacket and shackles. Jordan is nervous, but Joker tells him to watch the "show." Jordan applauds when Batman manages to get free of the straightjacket, but then realizes that Batman really is drowning. He grabs an axe and strikes the side of the tank, but Joker plucks the ax away before he can strike again, and reveals his true identity. Jordan grabs a seltzer hose and sprays Joker in the face, distracting him long enough to run out. As Joker chases after him, Batman pushes with his legs on the crack Jordan made, and manages to shatter the tank. Recovering his breath, he grabs his utility belt. ("All right, Joker – get ready for a little Bat-magic!") As Joker searches the amusement park for Jordan, Batman runs to the park's control room and throws the power switches, turning on the park's lights and attractions, to distract Joker and illuminate the area.
Joker catches up with Jordan, hiding on one of the roller coasters, just as Batman sees them. Joker starts one of the coasters and he and Jordan roll away on it. Batman jumps on the second as it starts, slightly behind the first. As the cars speed around the track, Joker throws grenades (shaped like Kewpie dolls) at Batman's coaster. The third lands right in the seat of the lead car, and the coaster de-rails in the explosion. However, Batman has managed to jump onto the rear of Joker's coaster. Joker lunges into the back. As they wrestle, he tries to squirt Batman with his boutonniere, but Batman ducks and kicks Joker off the coaster, causing him to fall with a scream into a lake far below. Looking ahead, he sees the coaster is heading for a break in the track, while Jordan is still in the lead car. Batman yells for Jordan to take his hand. Jordan hesitates for a moment, scared of Batman, then reaches, and Batman swings them both away just before the coaster goes off the rails and crashes.
At the mayoral mansion, Hill is sadly toying with his son's magic tricks, when Jordan calls, "Dad!" and runs into his father's arms. Having learned a lesson about how truly important his son is, Hill shushes Jordan's apologies and gives him "the best birthday hug" he's ever had. Over his father's shoulder, Jordan sees Batman smile and give him a thumbs-up. Jordan smiles back and returns it.