It starts out as a happy day for jellyfishing at Jellyfish Fields. SpongeBob and Patrick are catching jellyfish when SpongeBob says that if they would not sting, they would be perfect pets. A large alien jellyfish-like monster with large black eyes consumes a jellyfish, and then out of one of the tentacles spits a small pile of pink goo, which morphs into a jellion, a jellyfish with large black eyes. The jellion is commanded to engulf someone as the Jellion King spits out more jellions. Patrick goes to take a nap, with the jellion following him. As Patrick naps, the jellion stretches out its body so that Patrick is inside of it, then one of its tentacles spits out a tall pile of pink goo, with morphs into Jellion Patrick, which is Patrick but with large black eyes, same as the jellion. Patrick uses his blue mouth tentacles to pick up his net as SpongeBob catches the jellion. Jellions swarm into Jellion Patrick's net, and then Jellion Patrick gets an idea to turn everyone in town into jellion people by giving them jellions.
Later, SpongeBob and Jellion Patrick arrive at Squidward's house. Jellion Patrick stuffs his last jellion into Squidward's mail slot, then one second later, the jellion's tentacle spits out a tall light blue pile of goo from the window, which morphs into Jellion Squidward. Jellion Squidward and Jellion Patrick wishes SpongeBob a good night. SpongeBob sleeps cozily in his bed as the Jellion that originally captured Patrick attempts to capture him, but SpongeBob awakes with insomnia. As he watches a late movie, the jellion tries to capture him again, but SpongeBob throws popcorn into the jellion's mouth, and it starts choking on the popcorn, falling behind the couch.
The next morning, SpongeBob puts the Jellion into a cage as he leaves for work. It escapes, and consumes Gary as he sleeps and the plant next to him. His tentacle spits out another small pile of goo, which reveals a Jellion Gary-plant hybrid. The scene switches to the Krusty Krab which is completely filled with Jellion Bikini Bottomites, except for one man. The Jellion Bikini Bottomites order Krabby Patties "hold the mayo" until the man who wasn't a jellion person orders a Krabby Patty with extra mayo. Jellion Mr. Krabs comes and says that they were "out of the condiment called mayo," and he places a jellion on the guy's head. SpongeBob thought that was weird, because the Krusty Krab had a fresh wall of mayo in the freezer. SpongeBob told Jellion Patrick (who was operating the grill) that he was getting mayo for the guy's patty. Patrick turned his head around, his torso still facing the grill, yelled "NO MAYNAISE!!", then his mouth tentacles consumed the patty. SpongeBob went to the freezer and grabbed a box of mayo, then he looked through the peephole. It showed everyone in town, minus Sandy, hibernating in transparent green cocoons hanging from the ceiling, as the Jellion King commanded his jellions (on either side of him) to do stuff for him.
SpongeBob realized that something was very wrong. SpongeBob was ordered by Jellion Mr. Krabs to take a nap. SpongeBob flees from the Krusty Krab, with several Jellion Bikini Bottomites chasing him. SpongeBob is captured and kidnapped by Sandy. Sandy tickled him, and he giggled, showing he was not a jellion clone. They figured out that the jellion's weakness was mayo, since they ordered food with no mayo. They then exited with sunglasses with lens that looked like jellion eyes. Unfortunately, when joking in front of Jellion Mr. Krabs, SpongeBob elbowed Sandy, knocking off her sunglasses, and Jellion Mr. Krabs shouts "IMPOSTER," which knocked off SpongeBob's glasses. SpongeBob quickly grabbed a mayo sprayer and destroyed the entire town of Jellion clones. When a clone was destroyed, the real person of the clone awakened, and the cocoon they were trapped in disintegrated.
SpongeBob and Sandy find the hideout, containing every Bikini Bottomite cowering in fear. SpongeBob destroys one side of the Jellion King's jellions, but the mayo starts to freeze. SpongeBob tosses the mayo from the jar, but it freezes as well and lands in between the Jellion King's eye stems. Sandy turns up the heat, and the mayo melts, destroying the Jellion King. The remaining several jellions, not hit by mayo, then pop like bubbles. SpongeBob explained everything, which took eleven minutes. Patrick tastes the jelly and compliments it as relish. Soon, the goo remaining from the clones and jellions was sold as Evil Clone Jelly Relish for $4 extra than regular Krabby Patties. Patrick then orders a Krabby Patty with Jelly Relish, and then says "Hold the mayo." SpongeBob hears this and looks suspiciously at Patrick, and then the words "The End?" appear on the screen, ending the episode.
SpongeBob's spatula is cloned even though it is not alive. However, in "All That Glitters," SpongeBob's spatula is indeed alive and in "The Curse of Bikini Bottom," when he snapped his spatula in half, it had a ghost.
Running Gags: The Jellions saying "Hold the mayonnaise," and hissing at people.
The Jellions left the mayo boxes to cover their activities even though that is their weakness. SpongeBob could have gotten more mayo to fill the mayo dispenser before it froze.
The Jellion that Jellion Mr. Krabs puts on the customer was probably female since it had eyelashes. That was the only female Jellion in the episode since all the others did not have eyelashes.
Not counting Jellion Squidward, this is one of few episodes where Squidward appears but does not have any lines.
It is revealed in this episode that mayonnaise is another thing that can be added to a Krabby Patty as the Jellions ordered Krabby Patties without mayonnaise and the normal customer wanted extra mayonnaise.
Even still, the mayonnaise is still not shown to be put on a Krabby Patty in this episode.
The title card background is similar to the planet that SpongeBob and Squidward were teleported to in "SquidBob TentaclePants."
When the Jellion got Gary, Gary's shell colors swapped.
Because Gary was next to a plant when he got jellified, the clone was a hybrid of plant mixed with Gary.
Squidward was nott seen being cloned by the Jellion. Jellion Patrick just shoved it through Squidward's door and the goo is then spat out and morphed into Jellion Squidward.
When SpongeBob discovers the Jellion plot, he says "This is just like that movie I saw last night!" Since the episode is an obvious parody of the classic horror film Invasion of the Body Snatchers, it is likely that is the movie SpongeBob watched the night before.
This implies that the characters watch the same movies as we do. How that is possible is unknown.
In the original airing, this episode aired first, while "Are You Happy Now?" aired second (possibly because this segment was more heavily advertised). In later airings, the episode order is reversed.
On the DIRECTV info guide for this episode, Jellion is accidentally spelled as "Jellien."
Even though SpongeBob and Sandy eliminate all of the Jellions and save everyone at the Krusty Krab, the one character still never to be saved by SpongeBob and Sandy is Gary. He was never mentioned or shown again in the episode.
This episode is a parody of Invasion of the Body Snatchers.
When Patrick opens his mouth and tentacles come out, it is a possible reference to the Zalgo Garfield internet comics.
When SpongeBob first walked into the Jellion infested Krusty Krab and noticed a line of customers, there were two Jellion Clones in front of the normal fish. After the first Jellion Clone finished his order, the normal fish came up next, actually skipping the second Jellion Clone.
When Jellion Squidward tried to give SpongeBob a Jellion, it also had eyelashes just like the one Jellion Mr. Krabs gave to the customer. However, after SpongeBob leaves and Jellion Squidward still had the Jellion, its eyelashes are gone.
When SpongeBob first walked out of the Krusty Krab and bumped into Jellion Harold, he had his Krusty Krab hat on. However, in the next shot when SpongeBob was running from the group of Jellion Clones, his hat was missing. And finally, in the third shot after SpongeBob came from behind the rock, his hat was suddenly on again.
It is unknown why the Jellion Clones put the mayonnaise SpongeBob used in the freezer if it is their weakness.
When SpongeBob told Jellion Patrick (whom he thought was the real Patrick) that he needs a Krusty Krab hat, Jellion Patrick literally grew one on his head. When SpongeBob first mentions he is going to get mayonnaise, if one looks real carefully, Jellion Patrick's hat actually moves when he turns around saying "NO MAYONNAISE!" The hat shouldn't move since if Jellion Patrick grew it on his head, the hat should be a part of his body.
When Jellion Squidward pointed out the normal fish wanting mayonnaise on his Krabby Patty, the Patty was still on the tray. When SpongeBob took the Krabby Patty from Jellion Squidward to go and have mayonnaise put on it, the Patty was suddenly on a small plate.
Throughout the episode, Jellion Patrick's belly button keeps disappearing.
Just before Gary is engulfed by a Jellion, his shell inverts colors in between scenes.
This might be implying that Jellions have some level of difficulty maintaining the exact shape of an individual.
Gary was not seen in the freezer with the rest of the Bikini Bottomites hibernating in transparent green cocoons hanging from the ceiling.