SpongeBob SquarePants - Season 3 Episode 1 -

Mermaid Man drops his belt and SpongeBob finds it and tries to return it. But then a little mishap happens. He has shrunken all of Bikini Bottom, including Mermaid Man and Barnacle Boy. And then he shrinks himself and shrinks Bikini Bottom, and then Plankton comes back and is bigger than everybody.