Monsters, Inc. introduces viewers to the monsters' world, where they live and work. We meet Sulley and Mike, best friends who work at the Monsters, Inc. factory. In order to power their city, the monsters must use special 'doors' to enter the human world and scare children, collecting their screams to use as power. One night, Sulley's archenemy, Randall, works late and accidentally leaves a door on the factory floor. While running an errand for Mike, Sulley discovers the door, enters it to investigate, and comes out with a little girl, known through most of the film as Boo, hanging on his back.

Sulley and Mike frantically try to make a plan to send Boo back home before she destroys the whole power system of the city with her screams. While they try to care for her, they discover that Randall and Mr. Waternoose are involved in a...