From a distance, Mermaid Man's belt looks like an ordinary belt that is violet purple in color with the letter "M" that is light metallic blue in color (in Mermaid Man's early appearances, it is yellow similar to Aquaman's belt), but up close, it appears to have a bunch of hardware inside of it.
Role in episode
SpongeBob accidentally "steals" Mermaid Man's belt when he left it behind from running away from their freakishly eager fan, who is no other than SpongeBob himself. SpongeBob had decided to wear the belt and discovers the Shrinkray button, and he makes his own little bug tea party while still working by shrinking the Krusty Krab tables and food and feeding it to the bugs. He even shrinks his Krusty Krab employee hat.
Squidward finds him after getting disrupted by his work, and hence, threatens to call Mermaid Man and Barnacle Boy after he discovers that SpongeBob has the belt. SpongeBob gets upset by this after telling Squidward that if he tells Mermaid Man, he'll be kicked out of his fan club for good and gets so out of control with crying that he accidentally zaps Squidward with the Shrinkray. Worried that Mermaid Man will find out that he has his belt, he tries to find a "wise" friend and quickly resorts to Patrick.
Patrick mistakes the new mini Squidward for an action figure and unstoppably starts to play childishly with him. After Patrick suggests to flip the M upside down, SpongeBob accidentally zaps Patrick with the Shrinkray too, and then Sandy, then Larry, followed by everyone in Bikini Bottom. Everyone, except for Mermaid Man, Barnacle Boy, and Patrick, is mad at SpongeBob and start to attack his organs inside his holes. Then suddenly, SpongeBob for once comes up with a bright idea- to shrink Bikini Bottom, and so he does, making himself and everyone happy.
At the end of the episode, the bus arrives in large size and drops off Plankton, and humorously, he is confused with why the town is now just the right size for him, being small.
Role in video games
The Yellow Avenger
I can turn into a skyscraper!
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SpongeBob's Truth or Square
It is one of SpongeBob's memories.
According to SpongeBob, Mermaid Man has worn the belt for 65 years.