SpongeBob and Patrick are all set to leave for the jellyfishing convention in Ukelele Bottom that weekend. Realizing that he will not be able to take Gary, SpongeBob leaves Gary in Squidward's responsibility.
SpongeBob takes Squidward around his house, telling him that he will have to feed Gary for three days. There are very large cans of snail food with the words "morning", "afternoon" and "night" written on them, so that Squidward will be able to keep track. Eager to get rid of both of them, Squidward lies and says that he will do it to get SpongeBob and Patrick to leave. They do, and Squidward runs out of SpongeBob's house and sits in a lounge chair for the whole weekend, thus ignoring Gary. He soaks up plenty of sun, but Gary is left in the house, looking longingly in the direction where SpongeBob left, horribly neglected.
When SpongeBob gets back, Squidward suddenly remembers that he was supposed to take care of Gary. However, when he goes to get Gary, he finds him very sick, due to lack of food, water, and attention. Squidward, thinking he will be able to help, tried to give Gary some food. However, Gary is so weak from waiting for SpongeBob to come home, he is limp in Squidward's arms and will not eat. Squidward puts Gary down as he hears SpongeBob and Patrick saying goodbye to each other, and tries to force feed Gary. When he hears SpongeBob coming in, Squidward gives up and puts all of Gary's food into his mouth. He greets SpongeBob awkwardly, and explains that he was only checking on Gary.
Just as Squidward is about to leave, a smirk on his face, Gary slowly enters the room. SpongeBob realizes that something is wrong, and quickly calls the local veterinarian, Dr. Gill Gilliam. He comes over to SpongeBob's house, and gives SpongeBob a syringe full of snail plasma. SpongeBob asks Squidward to give Gary the plasma, as he is squeamish. Squidward reluctantly agrees and, while trying to inject Gary, accidentally injects SpongeBob with snail plasma causing SpongeBob to worry about the consequences.
Upon realizing that Gary only needed water, Squidward leaves telling SpongeBob not to worry. Sometime later, SpongeBob starts acting slow and somewhat like a snail, as well as eating Gary's food himself. When he goes to the bathroom, his eyes stretch out of his body and grow, his arms and legs disappear into his body, and his body reshapes itself. As a finished result SpongeBob has turned into a snail, becoming SpongeSnail. Afterwards, he goes over to Squidward's house to get help, but Squidward is terrified of the fact that SpongeBob turned into a snail because of his lousy aim and he throws the snail plasma away and he tries to reassure himself, but he hears SpongeSnail at the door and starts blocking the doors and windows and tries to run and hide from SpongeSnail. He chases him for help, but Squidward keeps running from his life thinking he's under attack which ultimately makes a mess of his house. Amongst the chaos, Squidward accidentally injects himself with the plasma, turning him into a snail as well, becoming SquidSnail. The ending has Gary, SpongeSnail and SquidSnail meowing to the tune of "Blow the man down" on top of a fence. Patrick, annoyed by this yells at them to clam up and throws a boot at SquidSnail. Gary and SpongeSnail continues to meow, but SquidSnail tries to meows the rest that ends after fading to black.
When Squidward gets in bed he says he hopes that he never sees a snail again, but in later episodes he seems fine with snails like in "That Sinking Feeling" he greets Gary and in "The Great Snail Race," he adopts a snail.
It is unknown how SpongeBob and Squidward recovered from this incident. It's possible that the snail plasma eventually wore off.
This episode might explain why Gary hates Squidward as he is the reason Gary almost dies.
SpongeBob (As SpongeSnail) out of both Gary and Squidward is the only one without a shell but his pants on the lump on his snail back which used to be his bottom could be considered a shell, otherwise he is considered a slug.
SpongeBob said that he had all of the cans of Gary's snail food marked, but it wasn't mentioned in captions. It's possible they forgot to add this line to the subtitle transcript.
The song that Gary, SpongeBob and Squidward meow is "Blow the Man Down."
Squidward doesn't seem to know about Gary's existence in this episode, even though he has interacted with him in earlier episodes most notably "Culture Shock" when he introduced Gary to the audience.
The narrator does not pronounce the "3 days later" card.
This is the first episode where that happens and it's also the first time card of series.
Curiously, in the Filipino dub, a narrator does pronounce the time card (in English)
When Squidward said "Eat! Eat! Eat! You've gotta finish this food!," Gary was lip syncing to what Squidward was saying.
When SpongeBob says "That's okay, I'm a lefty anyway" when his arm disappears, the arm that disappears was his left arm.
In this episode, the snails are seen sitting on a wooden fence. However, there are no fences near the three houses.
SpongeBob says he is a left-handed in this episode but in "You Don't Know Sponge," he says he is ambidextrous.
When SpongeBob played fetch with Gary, there was a watch on his arm, but when Gary returns the toy back to SpongeBob, it is gone permanently in the episode.
When SpongeBob says, "What a great friend you are, Squidward." the captions said, "You're a great friend, Squidward." This happens many times in SpongeBob episodes due to the fact that the dialogue in many episodes is mispronounced, the grammar is always fixed in close captioning.