SpongeBob and Patrick are given the task of pet sitting Sandy's pets when she is out of town. One of Sandy's pets is a small monarch caterpillar named Wormy. SpongeBob and Pat quickly befriend the caterpillar and they enjoy a day of play together, however; their fun is disrupted; overnight, when SpongeBob and Patrick are not at the Treedome, Wormy goes through quick metamorphisis and becomes a monarch butterfly. When they come back in the morning, Patrick and SpongeBob think their new friend was eaten by the butterfly. They go breserk as they try to trap the "monster." Unfortunately, Wormy (now as a butterfly) manages to escape the Treedome while flying inside a bubble. SpongeBob and Patrick begin to spread news of the terrible monster throughout Bikini Bottom. Mass chaos erupts and the whole town becomes destroyed and deserted.
Sandy arrives at the shattered town in a few days. She immediately finds Wormy and places him inside a jar. All the townsfolk cheer and celebrate the capturing of the monster.
I can turn into a skyscraper!
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The close up of the butterfly is actually the head of a buzzing horse-fly.
Wormy is able to complete the full transformation from caterpillar to butterfly in just one night. In real life, this is impossible, and the length varies with different species if butterfly (in this case, Monarch butterflies, which is what Wormy was, spend ten to sixteen days in the chrysalis stage when changing into a butterfly.)
Sandy closely lampshades this by saying "you weren't supposed to change 'till I got back."
MY LEG: Fred runs out of his house after seeing the newscast.
The warning poster SpongeBob and Patrick put up depict a Viceroy butterfly, while Wormy is actually a Monarch butterfly. Viceroys belong to a different family group of butterflies, and are smaller and less common than the Monarchs. The reason why Viceroys resemble Monarchs is due to Müllerian mimicry, in which two separate species that share a certain predatory defense mechanism (in this case, being nasty-tasting to birds) also share very similar warning patterns.
This was the first episode to feature Patrick's new design, featuring thicker eyebrows, instead of the usual "Z" eyebrows.
When SpongeBob is saying "We did it, Patrick!" during the destruction scene, a meteor can be seen flying down until it falls off screen in the background a few seconds later, a flash can be seen in the background, referencing the landing explosion.
In this episode while Birdy was singing, apparently its voice is the same to a European Robin in the 2001 documentary nature film Winged Migration.
The name "Birdy" might be a reference to a character from old McDonald's commercials named "Birdie."
Though Wormy eventually pupates into a Monarch Butterfly, he looks nothing like a Monarch caterpillar.
At the part where SpongeBob and Patrick ran away from Wormy along with everyone else, the town is destroyed. But when they reach the stoplight while being chased, the town was perfectly fine.
After Patrick's helmet broke, acorns fell in it. The water should've been displaced and overflowed, but it stayed at the same level.
It's impossible for SpongeBob to be wearing a cheerleader outfit without taking his helmet off.
After SpongeBob and Patrick finally lead the butterfly outside, they only open the first door. There are two doors, so the butterfly should've stayed between them, instead of making its way outside.
Nancy's dress changes from yellow to gray in one point where the Bikini Bottomites are being chased by Wormy as butterfly.
When SpongeBob begs Wormy not to eat him, his mouth didn't sync right.
The mouse in the snake's digestive tract should be in the middle of the snake's body, not at the snake's tail.
When Patrick warns everyone about the monster, his mouth doesn't sync right.
After Mr. Krabs and Squidward run away from Wormy, two pairs of underwear can be seen, but Squidward doesn't wear pants or underwear.
When Mr. Krabs asks "What's the meaning of this, SpongeBob?!," Squidward's hat was missing.
In the part of the episode where it shows people running away, there was one yellow fish flying.
When Patrick was with the people holding up SpongeBob and Sandy, he doesn't have eyebrows. In another shot during the same scene, he doesn't have eyes. Also, SpongeBob's buck teeth and Sandy's left arm disappear for a split second. And he was floating and Sandy's badge was also gone.
After SpongeBob and Patrick trap the butterfly in the entrance of Sandy's treedome, Wormy somehow made it outside the front door and headed for the Krusty Krab.
When SpongeBob and Patrick first discover Wormy as a butterfly, they both run in different directions, but end up at the same table in the middle of Sandy's treedome.
When the final shot of Sandy was shown before she leaves the treedome, she was wearing her spacesuit and helmet, but in the previous shots, she was in her bra and bikini.
The bubble that SpongeBob blows to catch the butterfly was impossible to make because his helmet was filled with water on.
Fred's eyes turn gray while carrying Sandy and SpongeBob at the end.
In the scene after the citizen that watched the news cast jumped through his wall, Billy is flying around.