S05E29 - Mermaid Man Vs. SpongeBob
Synopsis
When Mr. Krabs releases a new Mermaid Man and Barnacle Boy promotion, Plankton gets an idea to use Mermaid Man and Barnacle Boy to run Mr. Krabs out of business. He hypnotizes them to convince people not to eat there anymore, so it's up to SpongeBob to turn them back to normal.