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 metamorphosis 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 berserk 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.
This marks the 50th episode segment of the series.
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.
There was a deleted scene in the original airing where SpongeBob was eating pie.
This episode was banned in some countries, due to the butterfly close-up frightening young viewers.
At the beginning Patrick whistles, and SpongeBob says: "I didn't know you spoke bird!," and Patrick says "No, that's Italian." In the Italian version, he says that it is Spanish.
The Krusty Krab cash register boat is driveable as seen in this episode.
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 run away from Wormy along with everyone else, the town is destroyed. But when they reach the stoplight while being chased, the town is perfectly fine.
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 did not sync right.
When Patrick warns everyone about the monster, his mouth does not sync right.
After Mr. Krabs and Squidward run away from Wormy, two pairs of underwear can be seen, but Squidward does not wear pants or underwear.
When Mr. Krabs asks "What's the meaning of this, SpongeBob?!," Squidward's hat is missing.
When Patrick is with the people holding up SpongeBob and Sandy, he does not have eyebrows. In another shot during the same scene, he does not have eyes. Also, SpongeBob's buck teeth and Sandy's left arm disappear for a split second. And he is floating and Sandy's badge is also gone.
When the final shot of Sandy is shown before she leaves the treedome, she is wearing her spacesuit and helmet, but in the previous shots, she was in her bra and bikini.
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.
After SpongeBob and Patrick finally lead the butterfly outside, they only open the first door. There are two doors, so the butterfly should have stayed between them, instead of making its way outside.
SpongeBob should not have been able to blow the bubble, which captured Wormy, as he was wearing his helmet on.