SpongeBob and Patrick are given the task of pet sitting Sandy's pets while she is out of Bikini Bottom. 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" by disguising Patrick as bait. 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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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.
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 of 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."
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 is the first episode to premiere in 2001. It also the very first episode to air in the 2000's, 21st century, and the 3rd millenium. However, it has a 2000 copyright date, which means technically this episode and Patty Hype were made in the 90's.
This marks the 50th episode segment of the series.
When SpongeBob is standing on a treasure chest, warning people about the butterfly, the word "Dogs" can be seen on a building in the background.
MY LEG: After seeing a butterfly that is attacking Bikini Bottom on the newscast, Fred runs out of his house and falls on the ground.
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 for a few seconds later, a flash can be seen in the background, referencing the landing explosion.
This episode was banned in some countries, due to the butterfly close-up frightening young viewers.
The Krusty Krab cash register boat is drivable as seen in this episode.
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.
Though Wormy eventually pupates into a Monarch Butterfly, he looks nothing like a Monarch caterpillar.
When Wormy's face is viewed as a close-up in butterfly form, the wrong insect is shown. As Wormy is a butterfly, there should have been circular black eyes and a proboscis (tongue). Instead, we see enormous rainbow colored shimmering eyes and sharp mouthparts, distinctive to that of a Horsefly. Furthermore, butterflies do not make buzzing sounds, which, again, is a characteristic of flies.
When SpongeBob begs Wormy not to eat him, his mouth did not sync right.
SpongeBob should not have been able to blow the bubble, which captured Wormy, as he was wearing his helmet on.
After SpongeBob and Patrick finally lead the butterfly outside, they only open the first door. There are two doors in the treedome, so the butterfly should have stayed between them, instead of making its way outside.
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 Patrick warns everyone about the monster, his mouth does not sync right.
In the scene after the fish citizen that watched the newscast jumped through his wall, Billy is flying around.
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 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.
Fred's eyes turn gray while carrying Sandy and SpongeBob at the end.