Sandy Cheeks is taking a trip somewhere, so she leaves SpongeBob and Patrick in charge of her many pets. Their favorite one is a caterpillar named Wormy. They play with him for a long time. They do not want the day to end, because they want to play more with Wormy. SpongeBob gives him a 'best friend ribbon' and he and Patrick promise to come back the next morning.
Overnight, Wormy turns into a butterfly. The following day, SpongeBob and Patrick find the butterfly in Wormy's jar. They let it go, and then concluded that it must have eaten Wormy (they do not realize that it actually is Wormy having gone through metamorphosis). They thought the butterfly was a monster. They start panicking and set up traps, but none of their plans work, until SpongeBob blows a bubble to trap the butterfly. It goes out of Sandy's Treedome and heads towards the Krusty Krab, where Squidward Tentacles is about to take a picture of Eugene H. Krabs with his first dollar. SpongeBob and Patrick go through the 'secret entrance', and hide inside the boat/counter. They jump in the picture, nocking Mr. krabs over, also ruining the picture. They tell them about the 'monster', but Squidward and Mr. Krabs think it is harmless. They look closer, and see a frightening buzzing creature (an extreme close-up of a butterfly), and run away. SpongeBob and Patrick think that the butterfly has eaten them. They then decide it 'ate three of their frinds, so they need to warn Bikini Bottom. It is only then that the destruction starts, with a conflagration similar to The Great Fire of London. It is not until Sandy gets back that the terror/tragedy ends, she knew what it was and they all thought she saved the day!