It's April Fool's Day in Gotham City, and the morning starts off bright and clear. However, the ominous sounding music lets us know that's not going to last for long. Sure enough, a trash barge passing beneath the Gotham River Bridge is emitting a foul green cloud that washes over the vehicles on the bridge. At the helm of the barge is a large clown, which means that the Joker is running this little scheme. One of the vehicles passing over the bridge is a Dinks security truck. The driver wrinkles his nose at the smell of the barge, and then suddenly starts laughing. In fact, he starts laughing so hard that he loses control of his truck, narrowly missing two painters who are repainting a section of the bridge. The painters yell after the truck driver, but then they start laughing as well. Meanwhile, the bridge is being raised for the passing barge, and the security driver is heading right for it. Before the truck goes over the edge, the driver bails out, and the empty truck sails into the river.
At Wayne Manor, Bruce Wayne is getting ready to start his day, but obviously he had a rough night of crime fighting earlier, and he cuts himself shaving. Alfred tries to comfort his master by letting him know that he's drawn him a bath. When Bruce goes to the tub, however, he finds the tub empty. Alfred holds up a drawing of a bathtub, and wishes Bruce a April Fool's day. Ah, Alfred, you're such a riot.
While Bruce is in the shower, he has Alfred turn on the radio so that he can catch up with the going-ons in Gotham City. Sure enough, there's trouble afoot. Traffic has stopped due to an overturned trailer of eggs, and there's been a series of near-fatal car accidents all along the river, since hundreds of Gothamites have just started laughing hysterically. Bruce quickly realizes that the Joker is at work here.
Back at the river, we see a submarine beneath the trash barge watching the security truck sink to the bottom of the river. Two men in scuba gear leave the submarine and break open the back of the truck. Inside are numerous bags of money, which they begin grabbing. Inside the submarine, the Joker is primping himself in a mirror, getting ready to make his grand debut in Gotham. He has the news radio playing as well, and we hear that the laughing attacks are moving toward the financial district. The news gets cut short at that point, however, as the newscaster succumbs to the laughing gas. "That's my cue," says the Joker.
On the streets of Gotham, an electronic banner warns that the stock market has fallen 20 points. Things are about to get worse, because the Joker's trash barge is pulling into dock. A panel opens up beneath the trash and Joker and his henchmen come out, pushing shopping carts. Everyone on the street is laughing, so the Joker's men have no trouble stealing their purses and wallets. The Joker stops by a Jewels "R" Us store and busts open a store window right in front of a cop, who offers no resistance.
Meanwhile, Batman has sent out a small weather balloon to take a sampling of the gas. The Batcomputer quickly discovers that the gas is coming from the riverfront district, and determines that "lengthy exposure to the gas will result in permanent insanity." Batman doesn't like the sound of that, and the situation is about to get even worse. He calls for Alfred to bring him his hex key, but Alfred tells him to "fetch it himself." Batman thinks this is just another one of Alfred's poor excuses of an April Fool's joke, but suddenly he hears a sharp crash from upstairs. He races into the library to find Alfred smashing everything in sight. The windows are open, and the Joker's gas has been drifting inside. Batman quickly puts his gas mask on, but it's too late for Alfred, who falls to the floor after breaking a Ming vase. Batman tries to calm Alfred down, with the dooming analysis of the Batcomputer echoing in his mind. If he doesn't stop this attack soon, Alfred will be permanently insane!
The Bat Boat races across the river, heading to the scene of the Joker's mayhem. Summer Gleeson is on hand, equipped with a gas mask, to report on the imminent collapse of the stock market. "The only things gaining now are the laughingstocks," Joker jests, but it seems he spoke too soon. The barge begins to shake violently, and Joker races over to his periscope. Taking a quick look around, he spots Batman, who punches the periscope, knocking Joker backwards. The Bat Boat now has the barge in tow and is leading it away from the city. Joker leads his men up to face off against the Batman.
Batman has no trouble taking care of the first two of Joker's henchmen, but the Clown Prince of Crime has another trick up his sleeve. The third clown henchman - the huge barge helmsman - rips loose the tow cable, and the Bat Boat goes speeding off on its own. He then comes over to attack Batman. None of Batman's attacks have any effect, since Joker's Captain Clown is a robot! The Captain whirls Batman around, making him dizzy and disoriented and then drops him into a steel drum Joker wheels over. Once inside, Joker clamps the lid down, trapping Batman inside. He then pokes several holes into the drum with a dagger, narrowly missing Batman with each strike. Finally, Captain Clown picks up the drum and tosses it into the river. All the holes Joker made in it quickly start letting in water, and the drum sinks into the river.
Joker sends the barge back towards Gotham while Batman continues to sink. Inside the drum, Batman has been completely submerged. He uses the homing beacon on his utility belt to call the Bat Boat to him, which submerses into the water to reach the drum. Once next to it, the Bat Boat unleashes its lasers to slice open the side of the drum, freeing Batman.
The Joker has reached the Ace Disposal Plant, and he's refilling the barge with his toxic laughing gas. Batman arrives on the scene, and tosses a throwing star through the hoses, dousing Joker's henchmen instead. This time, when the henchmen leap at him, Batman rips their masks off, exposing them to the gas and removing them permanently from the fight. That won't work with Captain Clown, so Batman picks up a steel pole and bashes the clown with it. All it seems to do is slow him down, so Batman races up a trash heap towards the massive trash compactor. The robotic clown snags Batman's cape, forcing Batman to stop and bash him in the head with the pole some more. Batman finally rips his cape loose and gets on top of the compactor. As Captain Clown clambers up to join him, Batman smacks him with the pole again, breaking loose his clown mask to reveal the robot machinery beneath. Batman knocks the stunned android into the compactor, but he only has a moment's respite before Captain Clown begins to climb back out. Batman pulls the lever to start the compactor, crushing Captain Clown. "You killed Captain Clown. You killed Captain Clown!" Joker screams as a Captain Clown cube exits the compactor. Joker retaliates by dropping a load of junk onto Batman and then hops on top of the Captain Clown cube, which is riding the conveyer into the Disposal Plant.
Batman gets out from beneath the junk and races after the Joker. Batman has almost caught up with the Joker when the clown grabs a hold of a hanging cable and swings away, while Batman is stuck with the cube, sliding towards a molten pit! Batman dives off the cube and grabs onto a bucket traversing the pit. He has barely climbed into the bucket when the Joker starts pushing massive crane hooks into it, trying to knock Batman loose. Batman leaps onto the second of these hooks, and swings back to Joker. Joker simply pulls a lever, which sends the hook - and Batman - hurtling back towards the molten pit. Batman jumps onto a nearby set of massive gears and rides them back up to the catwalk where Joker is.
Or rather, was, because Joker's run off again. Batman spots him racing towards a trash tube and chases after him. Both leap into the tube, but Joker again grabs onto a cable at the end and swings to safety. Batman tries to slow himself down, but there's nothing to grab onto and so he goes flying off the edge, towards the incinerator below. Luckily, Batman spins around at the last moment and grabs onto the end of the tube. Joker presses a button to release a load of trash into the incinerator tube. Batman is struck by this garbage, knocking him loose from the tube, but he narrowly avoids a fiery death by flinging a grappling hook up and around the catwalk Joker's on. Batman swings across the pit and confronts Joker, who tries to escape by flinging a razor-tipped playing card at Batman, who easily evades it. Joker backs away, but has secretly released another card from his sleeve. When he throws it this time, however, Batman simply catches it. Joker turns to run away, but trips himself on a cable and winds up dangling above the incinerator. Batman considers leaving him there for a moment, but then pulls the Joker up.
Back at Wayne Manor, Bruce returns to the library after taking his eighth shower to wash away the smell of garbage. Alfred is back to normal, but is feeling bad about the destruction of that Ming vase. Bruce tells him, "It's okay, Alfred. I'll just take it out of your salary for the next couple of years." Alfred resigns himself to that, and then discovers that this is Bruce's own poor version of an April Fool's joke.