Jimmy and Timmy’s experiment could win them first prize in the annual science fair.
The episodes opens at the Special Education department in the elementary school. It is announced that there will be a Special Ed Science Fair. Nathan and Mimsy go to the front of the classroom and present their idea for their science fair project to the whole class. Nathan plans to get "bitches" to like them by winning the science fair. The scene shifts to the girls at lunch in the school cafeteria. The girls talk about Heidi's condition since she went back to Cartman. Heidi appears and sits with the girls, having gained a lot of weight since her last appearance. Immediately after, it is revealed that along with her appearance, Heidi's attitude changed too. She suspects that the girls were talking about her again, and calls them "judgmental bitches". Wendy voices her concerns to Heidi. Heidi snaps at her and the other girls, asking if she could eat her lunch without them "shitting all over her."
Back at the special education department, Nathan and Mimsy go to brag about their project idea to Timmy and Jimmy. Timmy and Jimmy were working on their own project about water bears. Nathan laughs at them, not knowing what water bears are. Jimmy offers to show them. Nathan looks through their microscope and is shocked to see that they actually exist. Nathan asks them how they learned about water bears. Jimmy tells them they learned about them from Octonauts. Nathan tells them that their science project has no experiment. So Jimmy and Timmy show Nathan how they've trained the water bears react to different sound waves, discovering they react the best to Taylor Swift's music. Nathan is shocked and becomes jealous of Jimmy and Timmy's project.
Later, Mr. Mackey calls Heidi to his office to talk to her about how she'll be transported to the school that Saturday to judge the special needs Science fair. Heidi tells Mr. Mackey that she won’t be judging the science fair, not wanting to go to school on a Saturday. Mr. Mackey tells her she volunteered a month ago. He notices Heidi's attitude has changed and asks her what's wrong. She snaps at him, thinking he was calling her fat, and claims she is healthier than everybody in the school. Mr. Mackey tells Heidi that she is going to judge the science fair anyway, annoying Heidi. Later that night, Nathan and Mimsy break into the special education classroom. Nathan attempts to sabotage Jimmy and Timmy's science fair project by killing the water bears. Mimsy asks Nathan why he wants to sabotage their project. Nathan tells him that Jimmy is a "cockblocker", claiming that whenever Nathan tries to "score chicks", Jimmy one-ups him. Nathan proceeds to electrify the water bears with a hair curler.
That following morning, Wendy, Theresa, Isla, and Heidi wait at the bus stop. Heidi complains that she is being forced to judge the special needs Science fair, saying she's basically a slave. Wendy argues with her, saying she’s not exactly being treated like a slave, to which Heidi responds that she isn’t being paid to judge it, which essentially makes her a slave. She then speaks in a stereotypical African-American accent, pretending to be enthusiastic about judging the science fair to her slave masters, which angers Theresa. Heidi tells Theresa not to go agro on her just because she's angry that her family lives in a trailer. Theresa tells her that she doesn’t live in a trailer, and that she lives in a "tiny home" to put a smaller footprint on the environment. Heidi argues with Theresa over her living accommodations, which upsets Theresa, who says lots of progressive people live in homes like hers, and that there are five others on her block. Heidi tells her that she lives in a trailer park which makes Theresa yell at her in denial. Wendy tells Theresa to just ignore Heidi, but Heidi turned things around and acted like Wendy was talking to her, calling Theresa a bitch. Isla tells Heidi that Wendy wasn’t talking to her, which makes Heidi laugh and jokingly embellish Isla. Cartman appears, asking Heidi how she’s been, which irritates her.
Later at school in the hallway, Nathan walks up to Nelly, Bebe and Nichole. He attempts to flirt with them and hands Nichole a flyer, informing the girls of the science fair. The girls look worried, and accept Nathan's invitation. He walks away with Mimsy, claiming the girls were excited to see him and that they’ll be all over them once they win the science fair. One of the special needs kids appear and tell Nathan that he and Mimsy must hurry to the special education classroom. He tells them that Jimmy and Timmy's water bears have changed. They go to the special ed classroom to see everyone gathered around Jimmy and Timmy's project. Jimmy tells Nathan that overnight the water bears have evolved to learn the hokey pokey, irritating Nathan. Meanwhile, in the school hallway, Heidi and Cartman ask Butters to judge the science fair. Butters tells them that he can’t because he’s going to attend Clyde's birthday that same day. Stan and Kyle appear, asking why Heidi wouldn’t want to support the special education students. Cartman and Heidi both tell Kyle to shut up. Heidi tells them that nothing beneficial to society could ever come out of a special needs Science fair.
A helicopter appears just outside of the school. A group of government men ask PC Principal if they could help aid Jimmy and Timmy in their special needs Science fair project, dramatically claiming that it could potentially save all of them. At recess, Heidi complains that the special education classroom is overfunded, and later makes an announcement in the school auditorium that if the school continues to overfund the special needs classroom, that they will all be poor and live in trailer homes like Theresa. Cartman tells Heidi to get back on track, which pisses off Heidi who yells at him for not supporting her and being an asshole. She finishes up her speech, telling everyone that if the problems she addressed persist she will see to it that the school administration are fired for discrimination. Mr. Mackey tells PC Principal that Heidi is a lot like Cartman but with the ability to follow through, which spooks both of them. Meanwhile, in the special education classroom, Mimsy tells Nathan that if they hadn’t attempted to sabotage Jimmy and Timmy's project, they wouldn’t have made them so popular. Nathan realizes that they should have credit for advancing the water bears since they were technically the ones responsible for their social advancement. Nathan speaks to the group of government men who reveal to them that they’re actually the NFL, and that they think the water bears' psychological advancements can save them since their viewership has been tanking due to protests. They believe that the water bears can replace all the fans they’ve lost.
After experimenting further with the water bears with a FedEx box, Nathan and the government men discover that Jimmy and Timmy's water bears are different from other water bears. Then, PC Principal enters the classroom with Mr. Mackey, Heidi and Cartman. Mr. Mackey begrudgingly tells them that they have to cancel the special needs Science fair, and that they must shut everything down. The NFL men each pull out guns and hold everyone in the classroom hostage. Heidi yells for Cartman to do something about the NFL men. Cartman calls Heidi a 'bitch'. Heidi grows silent, and calmly tells Cartman to call her a bitch again, which intimidates him. Nathan makes a discovery looking through the microscope. The water bears have become NFL fans, sporting the gear of their favorite teams. The NFL becomes excited, but before they can duplicate the bears, Heidi grabs the bowl of water bears and runs out of the special needs classroom. The armed NFL men, the special education students, Nathan, Jimmy, Timmy, PC Principal, Mr. Mackey, and Cartman chase after her. Heidi leads them to the school entrance.
Out of breath, Cartman tells Heidi to calm down. The NFL men ask Heidi to please hand over the water bears. Heidi becomes upset and tells everyone that she is tired of people always mistreating her, referencing Cartman's abuse towards her in the past. Everyone ignores her, and tells her to give them the water bears. Nathan tells her to give him the water bears because he deserves all the fame and "bitches." The NFL tell her to give them the water bears because the water bears are a scientific anomaly. Cartman tells Heidi she needs to stop being so angry, and that everyone wants the old Heidi back. Heidi looks up at the other girls, then back at the bowl of water bears in her hand. Heidi drinks the water bears, killing them, and shocking everyone. Later, that Saturday, Heidi sits and watches cartoons with Cartman at his house. Heidi complains that there’s never anything to watch on Saturday except for kids cartoons. She forces Cartman to snuggle with her, and passes gas. Cartman moves away, but Heidi forces him to keep snuggling with her, and continues passing gas, horrifying him. Meanwhile, the NFL find Nathan and Mimsy's volcano from their science fair project, and reveal that with it, they’ve found hope.
AV Club gave "Moss Piglets" a "B-" rating saying: "Even though the core of “Moss Piglets” doesn’t fully succeed, Parker and Stone do manage to plant a seed that will likely have an enormous payoff in the future. Having ultimately stayed with Cartman after the events of last week, Heidi has begun to take on both his physicality and persona. She’s overweight, she’s narcissistic, she’s cruel (make that “crewl”). She’s even adopted Cartman’s detached “Oh, hey, babe. What’s going on” whenever he approaches."
IGN gave "Moss Piglets" a "8.6" rating saying: ""Moss Piglets" showcases South Park doing what it does best - telling a mostly standalone story about the kids getting into wacky shenanigans with a sprinkle of continuity and a dash of topical humor. While this episode dabbled in some familiar subject matter, it managed to deliver an entertaining take-down of the NFL and establish Heidi as a true force to be reckoned with. This episode worked as a great, self-contained romp, but there's potential to keep building from here."