20,000 Patties Under the Sea SpongeBob SquarePants : Season 5 Episode 34

Mr. Krabs opens a mobile underwater restaurant to compete with Plankton. SpongeBob and Patrick are tasked for the job and are unsuccessful with their first customers. They fall into an abyss and wake a sea monster up. The monster purchases Krabby Patties and Plankton comes along. Plankton is mistaken by the monster for a dessert (a chocolate éclair) and the monster chases him away.