Maggie Simpson spends the day in the Ayn Rand School for Tots. Marge Simpson drops off Maggie at the Ayn Rand School for Tots, where, after going through a security screening, she is classified as being of "average intelligence", upon which a guard carries her past the "Room for Gifted Babies" and puts her in the "Nothing Special" dreary corner. The playtime items are either taken away or eaten by the other babies. A butterfly then makes its way into the room as Maggie's nemesis, Baby Gerald, squashes and kills it on the wall with a mallet and draws a box around it with a crayon. A second butterfly also meets the same fate. Maggie finds a caterpillar and a pop-up book about the life cycle of the butterfly. Realizing that the caterpillar could also meet the fate of the first two butterflies, she tries to protect it from Gerald. The caterpillar later encases itself in a chrysalis and starts to transform. Once the newly formed butterfly emerges, Maggie tries to help it fly out of the window, but Gerald seemingly kills it by shutting the blinds on it as it attempts to pass through. Maggie dramatically mourns as she falls to the floor. Marge then arrives to pick her up, when it is revealed Maggie's scene was only a ruse to cover the truth: she had slipped her hair bow onto the windowsill and wore the butterfly on her forehead in its place. She then sets the butterfly free as Marge drives her home.

After Homer accidentally pollutes the town's water supply, Springfield is encased in a gigantic dome by the EPA and the Simpson family are declared fugitives. Homer adopts a pig who's run away from Krusty Burger after Krusty tried to have him slaughtered, naming the pig "Spider Pig." At the same time, the lake is protected after the audience sink the barge Green Day are on with garbage after they mention the environment. Meanwhile, Spider Pig's waste has filled up a silo in just 2 days, apparently with Homer's help. Homer can't get to the dump quickly so dumps the silo in the lake, polluting it. Russ Cargill, the villainous boss of the EPA, gives Arnold Schwarzenegger 5 options, forcing him to choose 4 (which is, unfortunately, to destroy Springfield) and putting a dome over Springfield to prevent evacuation. Homer, however, has escaped, along with his family. Can he stop the evil Cargill from annihilating his home town, and his family, who have been forced to return to Springfield?

In order to power the city, monsters have to scare children so that they scream. However, the children are toxic to the monsters, and after a child gets through, 2 monsters realize things may not be what they think. A city of monsters with no humans called Monstropolis centers around the city's power company, Monsters, Inc. The lovable, confident, tough, furry blue behemoth-like giant monster named James P. Sullivan (better known as Sulley) and his wisecracking best friend, short, green cyclops monster Mike Wazowski, discover what happens when the real world interacts with theirs in the form of a 2-year-old baby girl dubbed "Boo," who accidentally sneaks into the monster world with Sulley one night. And now it's up to Sulley and Mike to send Boo back in her door before anybody finds out, especially two evil villains such as Sulley's main rival as a scarer, chameleon-like Randall (a monster that Boo is very afraid of), who possesses the ability to change the color of his skin, and Mike and Sulley's boss Mr. Waternoose, the chairman and chief executive officer of Monsters, Inc.