SpongeBob and Patrick are given the task of pet-sitting Sandy's pets while she is out of town. One of the many animals left in their care is a small Monarch caterpillar named Wormy. Despite it requiring the least attention, SpongeBob and Patrick quickly befriend the caterpillar and enjoy a day of play together. As the two leave, they vow to have even more fun with Wormy the next day.
Overnight, Wormy goes through a speedy metamorphosis and becomes a Monarch butterfly. When they return the following morning, SpongeBob and Patrick search for their new friend but merely see an ugly winged creature instead. Upon opening the jar and finding only Wormy's discarded chrysalis inside, they come to the grim conclusion that this horrific beast has devoured Wormy and is now on the loose. They try to trap the supposed monster by disguising Patrick as bait. When this fails, SponegBob manages to encase the butterfly in a bubble and releases it outside the Treedome, saving Sandy's pets. However, now the butterfly is loose in Bikini Bottom, so SpongeBob and Patrick chase after it.
Beating the creature to the Krusty Krab, SpongeBob warns Mr. Krabs and Squidward, both of whom scoff at the tiny insect. However, with one good look at its face, the two are scared senseless and run away, their discarded undergarments leaving SpongeBob and Patrick to believe they were also eaten.
SpongeBob and Patrick begin to spread news of the terrible monster throughout Bikini Bottom. Mass chaos erupts with every tiny flap of its wings, and as a result of the people's hysteria, the whole town ends up destroyed.
A few days later, Sandy arrives back at the ravaged town. She spies Wormy, who was due to change after her return, and places him inside a jar. All the townsfolk cheer and celebrate the capture of the monster, Sandy believing it to be just because they missed her that much.
At the beginning, Patrick whistles, and SpongeBob remarks, "I didn't know you spoke bird," to which Patrick replies, "No, that's Italian, SpongeBob." 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 Wormy's case, Monarch butterflies spend ten to sixteen days in the chrysalis stage when changing into a butterfly.
Sandy 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 is the first episode to permanently feature Patrick's current design, featuring thicker eyebrows in a "Z" shape, instead of the original "M" shape.
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 millennium. 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.
In the Portuguese version of the episode, the song "That's What Friends Do" is instrumental.
In a Top 20 List by blameitonjorge, the butterfly close up of Wormy was placed at #2 of the Top 20 Disturbing Moments in Kid Shows, only beaten by several disturbing moments in Courage The Cowardly Dog.
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 does not sync correctly.
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 correctly.
In the scene after the fish citizen that watched the newscast jumps 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 a 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. In addition, he is floating and Sandy's badge is also gone.
Fred's eyes turn gray while carrying Sandy and SpongeBob at the end.