In celebration of it being Sunday, SpongeBob makes a sundae using unconventional ingredients because he does not have any ice cream left and he finds bananas and cherries to be boring. After using ketchup and onions, he decides that he wants to put peanuts in it. However, he does not have anymore, so he uses the peanut plant sitting on his bathroom windowsill. Afterwards, he finally eats the sundae.
It gives SpongeBob bad breath that causes everyone in Bikini Bottom to avoid him, excluding Patrick because he does not have a nose. Since SpongeBob does not realize that everyone is avoiding him because of his breath, he assumes it is because he is ugly after Patrick assures him.
Later in the episode, Patrick tries some of the sundae, causing SpongeBob to realize that it gave them both rancid breath.
Role in Stinky Swagger
SpongeBob must collect peanut onion sundaes in order to keep his bad breath.
In Battle for Bikini Bottom the sundae was originally going to be used as a power-up that would allow SpongeBob and Patrick to run faster and destroy stone tikis. However, this was scrapped before the game's official release.