SpongeBob and Patrick are playing Mermaid Man and Barnacle Boy. SpongeBob pretends to be Mermaid Pants and Patrick pretends to be Barnacle Star. They notice Squidward, who they think is Doctor Negative, and attack his vegetable garden and eat the vegetables. Squidward is about to clobber SpongeBob and Patrick, but he remembers he is running late for work.
At the Krusty Krab, Mermaid Pants and Barnacle Star are singing the "Mermaid Man and Barnacle Boy Action Theme," annoying Squidward. He tries to get them to stop talking about Mermaid Man and Barnacle Boy to no success. Squidward tells SpongeBob to take out the trash, but instead, they punch the trash, causing it to make the place dirty and all the customers leave after the incident.
Mr. Krabs comes out of his office and sees that SpongeBob and Patrick are playing Mermaid Man and Barnacle Boy. He shows Squidward his secret collection of comics books. Mr. Krabs then shows him his Captain Tightwad costume and dresses Squidward as Dr. Negative.
Now in costume, the two fight with Mermaid Pants and Barnacle Star. Mermaid Pants points out that "no superhero should fight a super villain unless that super villain has committed a crime." They go to a bank and Captain Tightwad and Dr. Negative pretend to rob it, until they get beat up by a security guard.
Mermaid Pants decides to skip to the end of the store and pick up evil supplies for the super villains. Now, Captain Tightwad and Dr. Negative have Mermaid Pants and Barnacle Star over a pool of boiling oil, but Dr. Negative plots to actually fry Mermaid Pants and Barnacle Stae.
The heroes tip over the lift and are set free, but Captain Tightwad and Dr. Negative get fried instead.
Mermaid Pants and Barnacle Star then fake drive into the distance.