The parallel city of Monstropolis is inhabited by monsters and powered by the screams of children in the human world. At the factory of Monsters, Inc., employees called “scarers” venture into children’s bedrooms to scare them and collect their screams, using closet doors as portals. This is considered a dangerous task because the monsters believe children are toxic and that touching them would be fatal. However, production is falling as children are becoming harder to scare and the company’s chairman Henry J. Waternoose III is determined to find a solution. The top scarer is James P. “Sulley” Sullivan, who lives with his friend and assistant Mike Wazowski and has a rivalry with the ever-determined chameleon-like monster Randall Boggs. During an ordinary day’s work on what is known as the “Scarefloor”, another scarer accidentally brings a child’s sock into the factory, causing the Child Detection Agency (CDA) to arrive and cleanse him. Mike is frequently ridiculed by the company’s clerk Roz for never completing his paperwork on time.
While working late at the factory, Sulley discovers that Randall left an activated door on the scarefloor and a young girl has entered the factory, much to Sulley’s horror. After a few failed attempts to put her back, he places her in his bag and hides when Randall arrives and returns the door to storage. Mike is at a restaurant on a date with his girlfriend Celia when Sulley comes over to him for help, but chaos erupts when the girl is discovered in the restaurant, and the CDA is called. Sulley and Mike escape the CDA and take the girl home, discovering that she isn’t toxic after all. Sulley quickly grows attached to the girl and names her “Boo”. The next day, they smuggle her into the factory and Mike attempts to return her through her door. Randall discovers that Boo is in the factory after seeing Mike in the newspaper with her. He tries to kidnap Boo, but instead kidnaps Mike by mistake.