The Shell Lodge Squad enters a world of monsters called Monstropolis. There, they learn that millions of years ago, the monsters once lived in disharmony with Mankind. But the humans thought the monsters were ugly, hairy or bald, had bad teeth, and smelled terrible, and chased them off their world. The monsters decided to find a new home with no humans to pick on them. So they swam out to sea and eventually came across an enchanted island where the food they ate made them look more and more scary looking. Realizing that their newfound apperance could be useful to them, the monsters used the world's magic to transport themselves to the human world and scare the humans sheetless. Then, thousands of years later, the monsters decided to use this act of scaring humans to cultivate their screams into a renewable energy source to power their magical home, now called Monstropolis. To do so, they used their island home's magic and shaped it with technology to create magic doors to come to the human world to scare humans, mostly children, to gather their scare energy. But there were a few monsters, mainly the founders of the CDA, who felt that humans don't deserve to have continued contact with monsters just to be scared for energy, and should be avoided outright. So they spread lies about humans being toxic to touch and being outright dangerous to keep their rule over Monstropolis secure.
Once the Shell Lodge Squad discovers this origin, they must make it their goal to try and patch up relations between the monsters and the humans, as well as try to end the energy crisis that was caused by the humans becoming harder to scare with news and movies dulling their reactions of fear. But they and their new friends, Mike and Sully, will have to hurry because if they don't stop Waternoose and Randall within 2 days, the Villain League will cause a Black Cloud to form, causing the Monsters Inc. factory to malfunction, and therefore will cause a chain reaction that will zap every human child, absorbing their souls, and killing them, allowing the Villain League to use these souls for Malefor's hunger...not if Waternoose's desire to use the Scream Extractor to just replenish the energy crisis gets in the way of Mirage's plans.
- Randall: You don't know how long I've wanted to do that, Sullivan!
- Patrick: Stop right there, fork-tongue!
- Randall: Go throw your stomach onto a clam, tubby!
- Patrick: (Record Scratch) Tubby? GRRRRGH! NOBODY CALLS ME TUBBY! (Attacks Randall, but misses, Randall chuckles while camouflaging himself) Come on out and fight, Chameleon! (Gets punched in chest by Randall, then whapped in the face) No! please, wait! (Randall beats him a few times, pushes him to the ground, picks him up, spins him around, smacks him back and forth on the ground a few times) Uhhhgh! (Randall picks him up by his pants) No! Please! Have mercy! (Randall kicks Patrick off in the distance) AAAAAAARRRRRGH!
- Randall: Now it's your turn, Sullivan!