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 continue to meow, but SquidSnail tries to meow. The episode ends after fading to black.
When SpongeBob's arm falls off, he doesn't regenerate his arm like he does in "Graveyard Shift."
This was likely because SpongeBob was unintentionally transforming into a snail.
Also when SpongeBob screams, his eyeballs turn into snail eyes, but in the next scene when SpongeBob says, "Gary, I'm fine." his snail eyes have eyelids on them.
This may be an animation error.
SpongeBob eats Gary's snail food in this episode. He does it again in "Missing Identity." The fact that he has stated to have never eaten snail food previously in "Missing Identity" indicates that this episode took place after that episode.
When SpongeBob's arm falls off, he says "It's OK, I'm a lefty anyway." despite the fact that it IS his left hand that falls off.
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.
This episode could take place after those episodes.
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.
If you reverse Patrick's line, "Will you clam up?", it sounds like he is saying, "All hail Squidward", which is a secret message.
SpongeBob said that he had all of the cans of Gary's snail food marked, but this wasn't mentioned in closed-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," which is what the show's theme song is based off of.
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.
This is the first episode to have a time card to mark how long time has gone by. In this case the time card reads "3 Days Later." However, the narrator does not pronounce it. Curiously, in the Filipino dub, a narrator does pronounce the time card (in English).
Oddly, Squidward threw the injection earlier in the episode and it made a breaking sound. However, it's perfectly fine at the end of the episode as he is injected with it.
In the episode "Squid's Visit," Squidward said that he has never went to SpongeBob's house, but in this episode, Squidward comes to SpongeBob's house to feed Gary. It is most likely that this episode took place after "Squid's Visit."
In the Polish dub, the episode's title is "Ślimaczy weekend" translating to "The Snail Weekend."
In the Croatian dub, the episode name is "Kad sam bio Slavko" translating to "When I was Gary."
SpongeBob transforming into a snail is very similar to David Kessler transforming into a werewolf from the 1981 movie, An American Werewolf in London.
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 Conch Street.
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 closed-captioning.