At Lard Lad Donuts, Chief Wiggum investigates a break-in involving the theft of the grease used to fry donuts. During the investigation, a creature composed of grease emerges from a sink, resulting in Wiggum's death. Mayor Quimby holds a press conference regarding Wiggum's loss and nominates Lou as the new chief of police. To help investigate it, he consults an expert named Moe, who explains that accumulated fat and grease have been combined to create a sewer-dwelling entity called Fatberg. He recommends shutting down the state fair until the situation is resolved, but Homer and Bart arrive to intervene with a bribe to keep the fair open to avoid financial loss.

Lou decided not to be his police chief if there is still the monster around there, so Quimby appoints Eddie as the police chief instead, marking the start of the fair state. It seems to begin quietly while Lou ranting about fried grease while the monster remains underground. Suddenly, there is a false alarm, involving Shauna and Uter, until the monster grabs Uter. Lou begins shouting for everyone to run as Fatberg starts attacking, growing larger as it consumes everyone, including Bart, where Homer tried to save him, but failed. Homer regrets his decision and comes up with a plan to stop the monster using a hat-a-pult, but in doing so, he sacrifices himself to destroy the monster in the process. After the monster is defeated, Lou and his fiancée head off to enjoy the ruins of the state fair.

Before the episode aired, a disclaimer revealed the following episode was lost found footage of the Krusty's Halloween Special. As the episode begins, Krusty welcomed all the kids (who are in costumes) as he is happy to be in his first Halloween Special but is worried that this special might not be as good as he thought. During the commercial break, Krusty is backstage as his producer came and warns Krusty that if this special doesn't do well, his rating might go down to 30 shares.

During the first skits, which parody The Exorcist, with Krusty as the mom, Sideshow Mel as Father Damien Karras and Mr. Teeny as Regan MacNeil as both Krusty and Mel threw pies at Mr. Teeny to get the demon out. However, after being hit by pies, Mr. Teeny escaped and broke character and he started attacking Mel and ripped off his face, causing the audience to scream in terror, with one of the camera men shooting Mr. Teeny's head off and landing on Ralph, which upon seeing his eyes now possessed himself.

Returning backstage, Krusty is told by the producer that the segment was so incredible, which made Krusty upset that his monkey tore off Mel's face (which luckily, he is still alive) but it seems that because of what happened, his ratings have improved and told him to continue this special. During the costume contest, Ralph's mouth opened up to reveal the Devil, who started causing havoc on stage, where he revealed that he came to collect from a deal he made. At first, Krusty believed that the deal was the same one that the producer mentioned a few times, but the Devil revealed that the deal wasn't made by the producer but rather Bart himself.

It was revealed that because of very popular and educational shows were coming after Krusty, Bart made a deal with the devil to have those shows cancelled in order to keep Krusty's show on the air and now it's time to collect as the Devil has summoned the corpses of the stars of the cancelled shows to drag Krusty to Hell, but Bart pleaded and asked him to take him instead, which they did follow by planning to teach him lessons from now on.

The scene then freezes where we see a much older Krusty talking to some Netflix producers to make his show now a streaming series, with the Devil trying to get the show to be on Netflix but they rejected the concept.

Earth was once a lush, green planet, occasionally interrupted by asteroids that wiped out the dinosaurs. Over time, the dinosaurs turned into oil, which eventually became plastic, leading to a world overwhelmed by plastic, a true plastic planet. Lisa is a visionary leader, dreaming of a sustainable future for humanity and reflecting on the days when plants thrived in natural soil. Milhouse and Skinner climbed out of a hole, claiming they had found dirt, but what Milhouse actually discovered was a magazine called Celebrity Dirt.

They felt frustrated as Milhouse was unable to find anything, Lisa told him to continue digging to find real dirt. Milhouse tried but found nothing left, so he began shaking the dump floor until they suddenly tumbled into the cave. They find themselves standing in front of the Kwik-E-Mart, stocked with long-lasting food and an assortment of seeds. However, Skinner is killed when Raiders arrive, led by their leader Bart, who has survived. They start fighting each other, but the voices of Homer and Marge are heard, reminding them to play nicely. After stepping outside, Homer explains that they survived by falling into radioactive waste filled with plastic, which transformed them into plastic themselves, and others became plastic as well.

Homer, Marge, and the residents of Springfield try to persuade them to turn plastic, but they resist and instead try to eliminate all plastic with fire and spicy Cheetos, even targeting their parents. With all food and other things gone, Bart and Lisa remain as the last humans. Maggie emerges from the Squishee machine after their parents perish. Bart tries to destroy Maggie, but Lisa decides to protect her, believing she may lead the next phase of humanity. In conclusion, it is revealed that one version of Maggie narrates the story, with the world now focused on the plastic Maggie.