- Gary the Snail
- SpongeBob SquarePants
- Patrick Star
- Squidward Tentacles
- Other Jellyfishers
- Purple Doctorfish (debut)
- Clam (mentioned)
SpongeBob and Patrick are all set to leave for the jellyfishing convention in Ukulele 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. Some time 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, sleeping and annoyed by this, yells at them to clam up and throws a boot at SquidSnail knocking him off the wall. Gary and SpongeSnail continue to meow, but SquidSnail tries to meow. The episode ends after fading to black.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ House of Horror - W. Merrick Farran [title card]
‣ Honolulu March - Hans Haider [opening]
‣ Aloha - Dick Stephen Walter [the next morning]
‣ The Achterhoek Dances - Jan Rap [Squidward at the door]
‣ Me for You - George Callert [Squidward not listening to SpongeBob]
‣ Cream Pie - Sage Guyton, Jeremy Wakefield [bus arrives]
‣ On the Beach - Kapono Beamer [Squidward sunbathing]
‣ Grass Skirt Chase - Sage Guyton, Jeremy Wakefield [Squidward tries to feed Gary]
‣ Funeral March - Frederic Chopin, Peter Lea-Cox [Gary's sick]
‣ Dramatic Impact 5 - Ivor Slaney [SpongeBob screams]
‣ Woe is Me! - Richard Myhill ["Squidward, something's wrong with Gary!"]
‣ On the Beach - Kapono Beamer [snail plasma]
‣ Cello Link (a) - Dick Stephen Walter [Gary coughs]
‣ Gator - Steve Belfer [Squidward tries to give Gary the plasma]
‣ Dream Sequence - Dave Hewson ["I don't know, I feel kind of funny."]
‣ Hawaiian Holiday - Sage Guyton, Jeremy Wakefield ["He just needed water?"]
‣ Twelfth Street Rag - Sage Guyton, Jeremy Wakefield [SpongeBob eating snail food]
‣ Curse of Evil 1 - Gregor F. Narholz [SpongeBob transforming into a snail]
‣ Esta Noche (a) - Hans Ehrlinger [Squidward in bed]
‣ House of Horror - W. Merrick Farran [SnailBob at Squidward's door]
‣ Hawaiian Link (a) - Richard Myhill [The moon shines]
• SpongeBob ScaredyPants - The Ghastly Ones [closing credits]
- As of April 14, 2017, The Internet Movie Database (IMDB) gives this episode a rating of 8.5/10 based on 346 ratings by users. TV.com users give this episode a 8.7/10 based on 200 votes.
- "I Was a Teenage Gary" was ranked #41 during the Best Day Ever event on November 9–10, 2006.
- There are 17 tikis on the title card.
- This episode is known to scare younger viewers because of SpongeBob's transformation. 
- In the Lights, Camera, Pants! video game, there is an art gallery with a painting that is based on this episode.
- 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.
- 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.
- This episode might explain why Gary hates Squidward as he is the reason Gary almost dies.
- 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 takes place after that episode.
- 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.
- 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 the Snail and in "The Great Snail Race," he adopts a snail.
- This episode could take place after those episodes.
- There was a rumor that in the original airings, there was a scene showing Squidward's transformation to a snail. Although, it has been proven to be fake.
- 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.
- The aftermath of the Snail SpongeBob chasing Squidward leaves Squidward's house a mess with the wreckage including a portrait of Squidward destroyed by the end of his clarinet.
- Coincidentally, the Snail Plasma item is caught on Squidward's nose a few seconds afterward.
- SpongeBob (as a snail) 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.
- The song that Gary, SpongeBob, and Squidward meow is "Blow the Man Down," which is what the show's theme song is based on.
- It is unknown how SpongeBob and Squidward recovered from this incident. It's possible that the snail plasma eventually wore off.
- 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).
- The narrator doesn't read the time card again in "License to Milkshake."
- This is the first episode in which Gary plays a prominent role.
- This was the first time SpongeBob is transformed into another animal rather than a shape.
- This was Steve Fonti's last time working on SpongeBob.
- This was also Chris Mitchell's last episode until Season 4 episode "Mermaid Man and Barnacle Boy VI: The Motion Picture," and his last as a storyboard artist.
- This episode and "Scaredy Pants," only tend to air in reruns in the United States on or around Halloween making this episode extremely rare and difficult to watch on TV, since they both aired during the Halloween season, and are Halloween-themed episodes.
- The same thing occurs with "Valentine's Day," and "The Paper," as they are Valentine's Day specials, and lastly with "Christmas Who?" and "It's a SpongeBob Christmas!" being rare episodes to see, with them being Christmas-themed specials.
- Despite this, Nickelodeon has shown this episode after their original movies because they have fifteen minutes to spare.
- In the episode "Squid's Visit," Squidward said that he has never gone 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."
- Squidward dislikes snails, such as Gary, but eventually gets a snail of his own in "The Great Snail Race," which makes it very unusual for a snail disliker.
- The title of this episode is a reference to the 1957 classic horror movie, I Was a Teenage Werewolf.
- 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 SpongeBob played fetch with Gary, there was a watch on his arm; but when Gary returns the fetch toy back to SpongeBob, it is gone permanently for the rest of the episode.
- When SpongeBob says, "What a great friend you are, Squidward." the closed-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.
- 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.
- However, in the Polish dub, SpongeBob says he's right-handed, correcting the error.
- In this episode, the snails are seen sitting on a wooden fence. However, there are no fences near Conch Street.