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 jellien, a jellyfish with large black eyes. The 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 Patrick naps, the jellien 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, which is Patrick but with large black eyes, same as the jellies. Patrick uses his blue mouth tentacles to pick up his net as SpongeBob catches the jellies. 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 Squidward'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. Jellien Squidward and Jellien Patrick wish SpongeBob a good night. SpongeBob 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 the popcorn, 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 and 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. The Jellien Bikini Bottomites order Krabby Patties "hold the mayo" until the man who wasn't a jellien person orders a Krabby Patty with extra mayo. Jellien Mr. Krabs comes and says that they were "out of the condiment called mayo," and he places a jellien 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 Jellien 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 MAYONNAISE!!," 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 Jellien King commanded his jelliens (on either side of him) to do stuff for him.
SpongeBob realized that something was very wrong. SpongeBob was ordered by Jellien Mr. Krabs to take a nap. SpongeBob flees from the Krusty Krab, with several Jellien Bikini Bottomites chasing him. SpongeBob is captured and kidnapped by Sandy. Sandy tickled him, and he giggled, showing he was not a jellien clone. They figured out that the jellien's weakness was mayo since they ordered food with no mayo. They then exited with sunglasses with a lens that looked like jellien eyes. Unfortunately, when joking in front of Jellien Mr. Krabs, SpongeBob elbowed Sandy, knocking off her sunglasses, and Jellien Mr. Krabs shouts "IMPOSTER," which knocked off SpongeBob's glasses. SpongeBob quickly grabbed a mayo sprayer and destroyed the entire town of Jellien 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 Jellien King's jelliens, but the mayo starts to freeze. SpongeBob tosses the mayo 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 explained everything, which took eleven minutes. Patrick tastes the jelly and compliments it as a relish. Soon, the goo remaining from the clones and jelliens 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.
Running Gags: The Jelliens saying "Hold the mayonnaise," and hissing at people.
The Jelliens 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 Jellien that Jellien Mr. Krabs puts on the customer was probably female since it had eyelashes. That was the only female Jellien in the episode since all the others did not have eyelashes.
Not counting Jellien 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 Jelliens 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."
Because Gary the Snail was next to a plant when he got jellified, the clone was a hybrid of plant mixed with Gary.
Squidward was not seen being cloned by the Jellien. Jellien Patrick just shoved it through Squidward's door and the goo is then spat out and morphed into Jellien Squidward.
When SpongeBob discovers the Jellien 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.
Even though SpongeBob and Sandy eliminate all of the Jelliens 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.
The episode is also based on the Martin Mystery episode "Attack of the Slime People," where the main characters find people in weird cocoons and the clones would disappear into slime when their weakness touches them.
When SpongeBob first walked into the Jellien infested Krusty Krab and noticed a line of customers, there were two Jellien Clones in front of the normal fish. After the first Jellien Clone finished his order, the normal fish came up next, actually skipping the second Jellien Clone.
When Jellien Squidward tried to give SpongeBob a Jellien, it also had eyelashes just like the one Jellien Mr. Krabs gave to the customer. However, after SpongeBob leaves and Jellien Squidward still had 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 real 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 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 Jellien Squidward to go and have mayonnaise put on it, the Patty was suddenly on a small plate.
When SpongeBob first walked out of the Krusty Krab and bumped into Jellien Harold, he had his Krusty Krab hat on. However, in the next shot when SpongeBob was running from the group of Jellien Clones, his hat was missing. And finally, in the third shot after SpongeBob came from behind the rock, his hat was 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!".
This might be implying that Jelliens have some level of difficulty maintaining the exact shape of an individual.
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 was 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.
When Sandy was tickling SpongeBob, he wasn't wearing his water helmet.