Oh, Professor Plankton, Door-to-door salesman.
— Professor Plankton, "Mermaid Man vs. SpongeBob"

Professor Plankton is an alter-ego of Plankton who only appears in the episode "Mermaid Man vs. SpongeBob."


Professor Plankton is Plankton with dark yellow long-sleeved shirt, a red bow tie, a white shirt underneath, a beige hat, a wooden cane, and a brown suitcase.


He visits Shady Shoals Rest Home to meet Mermaid Man and Barnacle Boy. He tries to convince them about his shampoo, by stating them that they need dandruff removal. They become convinced on the product, so he gives them dandruff control shampoo named "Prof. Plankton's Dandruff Control Shampoo" (revealed to be "Prof. Plankton's Mind Control Shampoo" later on).

He soon notices they are hypnotized by the shampoo, which then he reveals the shampoo to be "Prof. Plankton's Mind Control Shampoo." Since they are already hypnotized, it is too late for them to take notice of the effects.