It starts out as a happy day for jellyfishing at Jellyfish Fields. SpongeBob and Patrick are catching jellyfish when the latter says that if they would not sting, they would be perfect pets. Shortly after, 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 jellien, a jellyfish with large black eyes. Said jellien is commanded to engulf someone as the Jellien King spits out more jelliens. Patrick goes to take a nap, with the jellien following him. As the former naps, the latter 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 Jellien Patrick, whom is Patrick but with large black eyes, same as the jelliens. Patrick uses his blue mouth tentacles to pick up his net as SpongeBob catches the jellyfish. Jellies swarm into Jellien Patrick's net, and then Jellien Patrick gets an idea to turn everyone in town into jellien people by giving them jelliens.
Later at night, SpongeBob and Jellien Patrick arrive at Squidward's house. Jellien Patrick stuffs his last jellien into the latter's mail slot, then the jellien's tentacle spits out a tall light blue pile of goo from the window, which morphs into Jellien Squidward. This jellien and Jellien Patrick wish SpongeBob a good night. The latter sleeps cozily in his bed as the Jellien that originally captured Patrick attempts to capture him, but SpongeBob awakes with insomnia. As he watches a late movie, the jellien tries to capture him again, but SpongeBob throws popcorn into the jellien's mouth, and it starts choking on it, falling behind the couch.
The next morning, SpongeBob puts the Jellien into a cage as he leaves for work. It escapes, and consumes Gary as he sleeps, as well as the plant next to him. His tentacle spits out another small pile of goo, which reveals a Jellien Gary-plant hybrid. The scene switches to the Krusty Krab, which is completely filled with Jellien Bikini Bottomites, except for one man. Said Jellien Bikini Bottomites order Krabby Patties but "hold the mayo" until the man who wasn't a jellien person orders a patty with extra mayonnaise. Jellien Mr. Krabs comes and says that they were "out of the condiment called mayo," and places a jellien on the guy's head. SpongeBob thinks that is weird, because the Krusty Krab had a fresh wall of mayo in the freezer. He then tells Jellien Patrick (who is operating the grill) that he's getting mayo for the guy's patty. Patrick turns his head around, his torso still facing the grill, and yellows "No mayonnaise!" Then his mouth tentacles consume the patty. SpongeBob goes to the freezer and grabs a box of mayo, then looks through the peephole. It shows everyone in town, minus Sandy, hibernating in transparent green cocoons hanging from the ceiling, as the Jellien King commanded his jelliens (on either side of him) to do stuff for him.
SpongeBob realizes that something is very wrong. He is then ordered by Jellien Mr. Krabs to take a nap. SpongeBob flees from the Krusty Krab, with several Jellien Bikini Bottomites chasing him. Afterwards, he is captured and kidnapped by Sandy. The latter tickles him, and he giggles, showing he is not a jellien clone. They figure out that the jellien's weakness is mayonnaise since they ordered food with no mayo. They then exit with sunglasses with a lens that look like jellien eyes. Unfortunately, when joking in front of Jellien Mr. Krabs, SpongeBob elbows Sandy, knocking off her sunglasses, and Jellien Krabs shouts "imposter," which knocks off SpongeBob's glasses as well. The latter quickly grabs a mayo sprayer and destroys the entire town of Jellien clones. When a clone is destroyed, the real person of the clone awakens, and the cocoon they were trapped in disintegrates.
SpongeBob and Sandy find the hideout, containing every Bikini Bottomite cowering in fear. SpongeBob destroys one side of the Jellien King's jelliens, but the mayo starts to freeze. He tosses the mayonnaise from the jar, but it freezes as well and lands in between the Jellien King's eye stems. Sandy turns up the heat, and the mayo melts, destroying the Jellien King. The remaining several jelliens, not hit by mayo, then pop like bubbles. SpongeBob explains everything, which takes eleven minutes. Patrick tastes the jelly and compliments it as a relish. Soon, the goo remaining from the clones and jelliens is sold as Evil Clone Jelly Relish for $4 extra than regular Krabby Patties. Patrick then orders a Relish Patty 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.
When Patrick opens his mouth and tentacles come out, it is a possible reference to the Zalgo Garfield internet comics.
The episode is also based on Martin Mystery Season 2 episode 3 "Attack of the Slime People," where the main characters find people in weird cocoons and the clones would disappear into slime. their only weakness is mayonnaise
When SpongeBob first walks into the Jellien-infested Krusty Krab and notices a line of customers, there are two jellien clones in front of the normal fish. After the first jellien clone finishes his order, the normal fish comes up next, actually skipping the second Jellien Clone.
When Jellien Squidward tries to give SpongeBob a Jellien, it also has eyelashes just like the one Jellien Mr. Krabs gives to the customer. However, after SpongeBob leaves and Jellien Squidward still has the Jellien, its eyelashes are gone.
It is unknown why the Jellien Clones put the mayonnaise SpongeBob used in the freezer if it is their weakness.
When SpongeBob told Jellien Patrick (whom he thought was the real Patrick) that he needs a Krusty Krab Employee Hat, Jellien Patrick literally grew one on his head. When SpongeBob first mentions he is going to get mayonnaise, if one looks really carefully, Jellien Patrick's hat actually moves when he turns around saying "No mayonnaise!" The hat shouldn't move since if Jellien Patrick grew it on his head, the hat should be a part of his body.
When Jellien Squidward points out that the normal fish wants mayonnaise on his Krabby Patty, the patty was still on the tray. When SpongeBob takes the Krabby Patty from Jellien Squidward to go and put mayonnaise on it, the patty is suddenly on a small plate.
When SpongeBob first walks out of the Krusty Krab and bumps into Jellien Harold, he has his Krusty Krab hat on. However, in the next shot when SpongeBob is running from the group of Jellien Clones, his hat is missing. Finally, in the third shot, after SpongeBob came from behind the rock, his hat is suddenly on again.
Throughout the episode, Jellien Patrick's belly button keeps disappearing.
Just before Gary is engulfed by a jellien, his shell inverts colors in between scenes. The same error occurs in the episode "Bubble Buddy Returns" and in the episode "Treats!."
At the end of the episode, when SpongeBob walks out of the kitchen with a Krabby Patty, his pants are a different shade of brown than usual.
Gary is not seen in the freezer with the rest of the Bikini Bottomites hibernating in transparent green cocoons hanging from the ceiling, so it is unknown what happened to him and how he was back to his normal self by the next episode (Home Sweet Rubble).
When Sandy tickles SpongeBob, he isn't wearing his water helmet.