The Joker is sitting in his lair and his pet hyena named Giggles brings him the newspaper. Upon reading the article about a gas war, Joker becomes excited and he tells his pet that they are going to be rich.
Meanwhile in Wayne Manor, Bruce Wayne and Dick Grayson are playing a game of Pong. Bruce wins and tells Dick that now he's going to have to cook all the meals for the rest of the week. And Dick tells him he'll have to make his famous Peanut Butter and Sardine pie. Bruce is disgusted by the thought. Just then, the Bat-signal alerts them. They then rush of to the Bat-poles and slide down to the Bat-Cave.
Down in the Bat-Cave, Batman and Robin arrive and Batman notices something is wrong, he sees that the place has been wrecked. His tool boxes are open and there's junk everywhere. Just then, Bat-Mite flies in, and the Dynamic Duo are not happy to see him at all. Batman tells Bat-Mite that he thought he told him to stay in his world and stay away from Earth. Bat-Mite tells him he tried to, but he was way too bored. He tells him that they need him to help them fight evil. Robin tells him three is a crowd, but the small alien begs, telling him he only wants to help. But Batman tells him he's a royal pain. Bat-Mite tells him he can tell when he's not wanted, and he teleports away. Batman and Robin enter the Batmobile and head to Gotham City Police Headquarters. But what they don't realize is that Bat-Mite is a stowaway on the back of their car.
At the headquarters of the Gotham City Police Department, Commissioner James Gordon shows Batman and Robin a video tape that just arrived. They watch the tape, and on it, is Joker. He tells Batman and Robin that he's going to stage the biggest rip-off in the history of Gotham City...right under their noses...at seven o' clock. He also tells them that "Oil and water don't mix." The three of them are puzzled. But Bat-Mite, who up until now was hidden, tells everyone that what the Joker said about oil and water not mixing has many hidden meanings. The Gordon asks what it means. Then Bat-Mite says: "How should I know? He's the brains of the outfit." Batman tells the Gordon they'll do their best to stop Joker's plot, but they only have less than an hour, since it's already 6:05 PM. Batman and Robin rush off, Bat-Mite races after them, causing Gordon's paper work to fly all over him.
The Caped Crusader and the Boy Wonder drive the Batmobile back to the Bat-Cave. Upon their arrival, Robin fed the Batcomputer everything on the Joker from their crime file. Bat-Mite appears, and is about to give his opinion, but Batman walks over and picks him up and locks him in the closet. This angers Bat-Mite, and he uses his powers to teleport out of the closet. Robin tells Batman that there's not much information on his print out, and he assumes that their investigation has already "ran out of gas." Batman snaps his fingers. He then hands a newspaper to Robin and tells him to look at it. Robin then reads that a scientist by the name of Julius Erwin is going to demonstrate his new car that runs on water instead of gasoline, at seven o' clock at the Convention Center. Batman asks Robin what Gasoline is made from, and Robin tells him "Oil." And now they know that the Joker's clue was referring to that car. Batman then calls Gordon on the Batphone to inform him of their discovery. Meanwhile, unseen to Batman and Robin, Bat-Mite appears in the Batmobile and starts playing around, pretending to go after the Joker. But he causes the Batmobile to move out of control. Batman tells the Commissioner he'll call him back and he hangs up and runs after Bat-Mite. Batman uses his remote control to stop the Batmobile. He tells Bat-Mite he's going to stop the Joker without his help.
Not long later, it's about seven, and a full moon is in the sky, shining down over Gotham. At the Convention Center, the police are ready for the Joker, and Batman, Robin and the Commissioner are staking out in a police truck disguised as a laundry truck. Just then a man comes onto the stage and presents to the crowd the new car that runs on water. Batman points out that he has heard the Professor speak in the past and there's something different about his voice. Bat-Mite is also staking out the area from a tree. Back in the truck, Batman tells Robin to zoom in on the Professor's hand with camera number 3. He does so, and they see that the so-called professor is wearing a ring with Joker's image on it.
Batman and Robin race out of the truck. Meanwhile, Bat-Mite is observing them from a tree. Bat-Mite sees the two of them, and he shouts out: "Batman and Robin!" Robin is then irritated, because since he shouted, it alerted the Joker, who was disguised as the professor, and so the Joker jumps into the car that runs on water and drives off. The branch on the tree that Bat-Mite was standing on then snaps, and he falls, but Batman tosses a Batarang at Bat-Mite and he grabs onto it and glides down to safety. Bat-Mite kisses Batman for saving his life, and Batman tells him "We all have our weak moments."
Later, the Commissioner tells Batman that they found the real Professor tied up in a shed. The Professor tells the superheroes that they have to get the car back, because it hasn't been perfected, because under certain circumstances, it could explode, with the force of a nuclear bomb. But he can't remember what those circumstances are, because Joker bumped him on the head, causing him to get amnesia.
Later, Batman and Robin are talking with the Commissioner and the Professor at Police HQ. The Professor suggests that they alert the Gothamites that they have to evacuate. But Batman tells him that would only lead to widespread panic which would lead to chaos throughout the city. Bat-Mite then peaks his head out of the wastebasket. Batman then goes on to say that they have to find the Joker and the car before it's too late. They are baffled at where to start looking, when suddenly, Joker appears on TV, and Gordon recognizes that he's in the parking lot out back. Batman and Robin race after him. Bat-Mite also teleports away.
Meanwhile, out back, several police officers step into their cars to take off after the Joker, but the cars are filled with water. The Joker goes on with his broadcast and gets in the car and drives off, and tells the citizens of Gotham that he will sell Erwin's car to the highest bidder. Batman and Robin are standing on the ledge of a building and they drop a spring which blocks Joker's path as he's driving down the road. The Joker slams on the brakes, causing his tires to screech along the road. Joker looks up to see the two heroes, and Batman tells him to give them the car. He then turns the car around and drives off the other way, but Batman throws down foam and Joker can't drive through it. They climb down their Batropes, and Bat-Mite tries to help, but instead he accidentally frees the Joker's car from the foam allowing him to escape, right in front of the Gotham Cafe.
Meanwhile, back at police headquarters, the Commissioner is urging the Professor to think real hard so he can remember. He then notices a shaker of salt next to his sandwiches. He then remembers it has something to do with salt. Instantly the Commissioner informs Batman via the Batphone. Batman thanks the Commissioner for the information. The Professor goes on to tell them that if salt contaminates the car's power unit it will reverse polarity, which will cause the car to convert all of the water near it into energy, until it explodes. So the more water around, the bigger the explosion. Batman tells the Commissioner to get the Professor to the Gotham Waterfront immediately. Batman tells Robin to take the Batboat and he tells him he'll meet him there in the Batmobile. Instantly Robin races off in the Batboat. Bat-Mite then puts his hands over Robin's eyes and tell him "Guess who?" Robin can't see to drive the boat and temporarily loses control. He tells him to "Buzz off!" since he's caused so much trouble already. Bat-Mite then starts crying. Robin, feeling sorry for him, tells him he can stay on the condition that he sits still quietly and doesn't touch anything. Bat-Mite then says: "On James!" Robin reaches the dock, and he parks his boat next to it and tell Bat-Mite to stay put. Bat-Mite waits restlessly, and finally decides that there's really no harm in just taking one tiny little peak. He then teleports off of the boat. And looks for Robin at Pier # 7, but he is found by Giggles and he grabs him with his mouth and takes him to the Joker. They lock him in a steel cell which is impossible for him to escape. Meanwhile, outside, Robin is taking a look around, when suddenly he is caught in a fishing net by the Joker. Robin tells him the car could explode, and Joker says sure: "And I'm Santa Claus!" He then uses Robin's radio to contact Batman, and he disguises his voice to sound like Robin. Batman responds, thinking he's Robin. He told him he located the Joker at Pier 7. The Joker and Giggles laugh maniacally.
Batman catches up to the Commissioner's car and informs him and the Professor of what's going on. The Commissioner knows that there would be a lot of salt in the air due to the fact that the Waterfront district is next to Gotham Harbor, which is primarily salt water. The Professor tells him this makes the danger even greater. Batman races as fast as he can to the pier, and just then he looks in the sky and see Robin's Batsignal. He ejects the Batmobile in the Bat-Gyro. The Joker then sees the Batmobile and police car coming, but once it arrives he notices Batman isn't in the car. Batman alerts him from above and says: "Right here, Joker!" He then tangles the Joker in one the fishing nets. Batman takes the Professor to the car, where they see it is glowing, about to blow up. Robin and Bat-Mite are tied up in their next to the parked car. Batman pulls out his Bat-ray gun to cut the ropes holding Robin and Bat-Mite. Batman shuts down the fuse box inside, preventing the explosion, and then tells everyone that they have a lot of hot water...anyone for tea?
Back at Wayne Manor, Robin is serving his famous Peanut Butter and Sardine pie, and Bat-Mite tries it, and he teleports away to throw up. Batman then tells Robin to dish him up some of that, but Robin wonders why, since he doesn't like it. But Batman said that anything that gets rid of Bat-Mite is something he loves. He then tastes it, and is quite grossed out. They then start laughing.
During the Bat message segment, Batman and Robin explain that the Professor learned that it isn't what people think of you that matters, it's about doing what's right.
- This is the first appearance of Bat-Mite.
- This is the first appearance of Giggles. Giggles would later appear in DC Comics, but he was made up for this show.
- The Gotham Clock Tower appears in this episode.
A pest refers to something that brings about trouble by means of annoyance or other sorts of trouble. The primary trouble maker of the episode was obviously the Joker, but Bat-Mite could have easily fit the bill as well.