Mermaid Man and Barnacle Boy are entering the Krusty Krab, most likely for lunch, and upon realizing this, SpongeBob goes into an excited frenzy, bursting through a steel wall in order to get their autograph. The heroes run away from him, accidentally leaving Mermaid Man's utility belt inside. SpongeBob finds it, and tries to give it back, but they drive away in the Invisible Boat Mobile before he can do so. SpongeBob decides to wear the belt and play with it, thinking that he could hold on to it until after work. He discovers the "Small Ray," and starts shrinking the items in the kitchen. At the register, Squidward is about to hand a muscular customer his milkshake, when the commotion in the kitchen startles him, causing him to drop the shake on the guy's head, and the guy angrily punches him in the face before storming off. Squidward enters the kitchen to see SpongeBob serving tiny Krabby Patties to cockroaches and asks SpongeBob why is everything in the kitchen so tiny. Squidward discovers that SpongeBob has Mermaid Man's belt. Assuming that SpongeBob has stolen the belt, he plans to do the right thing and call Mermaid Man in order to get revenge, and tell on him. SpongeBob fears that if Squidward tells on him, Mermaid Man will be furious and kick him out of his fan club. In desperation, SpongeBob shrinks Squidward in order to stop him. Squidward demands SpongeBob to return him to normal size, but SpongeBob does not know how to do it, as he sees that the belt includes countless other buttons, as well as complicated switches and dials, with no idea what any of them do. SpongeBob tries to use the different buttons to make Squidward big, but none of them work. Not wanting Mermaid Man to find out due to how he might react, he leaves to find someone who can help him.
SpongeBob goes to Patrick for help, and Patrick mistakes Squidward for a "Squidward Action Figure" and starts playing with him, even after realizing that it is the real Squidward. Patrick then suggests to turn the "M" upside down, setting the belt to W for "Wumbo," which results Patrick coining the term. SpongeBob questions the validity of this, but after Patrick responds by stating the many uses of "Wumbo," SpongeBob is convinced by this idea and decides to test it out, but then he accidentally shrinks Patrick. Seconds later, citizens start coming and greeting to SpongeBob, making him shrink them so that they can't tell Mermaid Man. Eventually, SpongeBob has shrunken everyone (including his mother) in Bikini Bottom, and stuffs them into a jar. He then realizes that he has shrunken Mermaid Man and Barnacle Boy as well, and he apologizes and Mermaid Man goes into a long story to show he understands until the citizens of Bikini Bottom shout "JUST TELL HIM HOW TO UNSHRINK US!" in order to return to normal size. Mermaid Man asks him if he has tried setting it to Wumbo, which is enough to make everyone realize that he doesn't know how to unshrink them either. The people then form a mob and get out of the jar as they shout out "GET SPONGEBOB!" They go inside of SpongeBob's body and beat him (or rather, his organs) up. This continues for a while until eventually everyone says that everything is too big. This gives SpongeBob the idea to shrink Bikini Bottom itself, which satisfies the people, who see no discernible difference from their previous lives in living in a miniature Bikini Bottom. Finally, SpongeBob shrinks himself, and the tiny city begins celebrating. The closing scene shows Plankton getting off a bus, returning to Bikini Bottom from a vacation. He is surprised to see that he is now taller than Bikini Bottom.
In some airings, the scene where SpongeBob attempts to turn Squidward back to normal size by testing the various buttons on the belt is edited. The scene where Squidward is cut in half with scissors is edited out on occasion, as well as him choking and gurgling off-screen, most likely due to the violent nature. Also, on certain airings during the scene where SpongeBob zaps people in Bikini Bottom when they say "hi" to him, the flashes are cut out to possibly prevent seizures.
This is similar to a Pokémonepisode which resulted in 685 viewers having seizures.
In Indonesia, this episode is usually completely skipped, either due to ads or possibly because it is controversial.
In Indonesia, instead of saying "He, she, me, wumbo," Patrick says "She is a girl."
As well, instead of saying "What did you? For cryin' out. What did y'all do to me?," Sandy says "What did you do until it makes you cry? What did you do to me?"
This is the first episode where Mermaid Man's Belt is purple with a blue "M" instead of a yellow "M."
One of the people who get shrunken say "What's up, sponge?" This is a reference to Bugs Bunny's famous quote, "Eh, what's up, Doc?"
When SpongeBob is running to Patrick, one can see that SpongeBob is not wearing the utility belt in the first shot.
Even though there were a lot of cars parked outside the Krusty Krab, there were no customers inside it except for Mermaid Man and Barnacle Boy.
When Mermaid Man and Barnacle Boy drive off from the Krusty Krab, all the cars are gone.
After Mermaid Man uses the invisible boat alarm and says "There she is," he is just holding his pants, but when he gets to the boat, he is wearing clips on his pants, which he wasn't seen putting on.
The aforementioned clips holding up Mermaid Man's pants changed colors between shots.
Part of the Invisible Boatmobile where Barnacle Boy lands painfully is a gearstick, but then he sits on the seat without moving sideways.
When SpongeBob gives the Krabby Patty to the cockroach, the holes on his spatula disappear.
When SpongeBob first shrinks Squidward and Patrick, they are comparable in size to action figures (roughly 6-8 inches). When he shrinks the rest of the townspeople, they are about the size of jelly beans. When he shrinks the whole town and then himself, Plankton is taller than the tallest building, and taller than SpongeBob was before he shrunk, despite he should in realistic proportions be the same size as everyone else.
Mermaid Man is overweight, but his pants do not fit him when he does not wear his belt.
When SpongeBob shrinks Patrick, he has the emblem on the belt set to "Wumbo," but afterwards, it is turned right side-up again.
When SpongeBob shrinks Sandy, Squidward and Patrick's eyes are black.
SpongeBob has difficulty closing the jar with everyone trying to fit in it, but when Mermaid Man mentions setting the belt to "Wumbo" and everyone yells "What?!," it appears that they don't quite fill up the jar to the top, and he has plenty more room. This also happens when he cries "You don't understand!"
SpongeBob can't get the citizens of Bikini Bottom back to normal, but in "Mermaid Man and Barnacle Boy," SpongeBob mentions a ray gun that made things grow six times their normal size. It's excusable that SpongeBob was so scared that he forgot about it.
In some shots of when the townspeople are beating up SpongeBob's inside, SpongeBob has bones. However, SpongeBob is an invertebrate.
When everyone pops out of SpongeBob, Sandy's head is missing.
When SpongeBob shrinks himself, Mermaid Man's belt is changed into his normal pants.