Monsters, Inc. is the power utility in Monstropolis, a monster-populated city; the company uses the power of human children screams scared by its employees, who enter their rooms via closets linked to special doors on the company’s “scare floor”, to generate the city’s power. However, due to children becoming desensitized, the company finds itself struggling to meet the power needs of Monstropolis.
The company’s top scarer is James P. Sullivan or “Sulley” (John Goodman). One day, while turning in paperwork to the dispatch manager Roz (Bob Peterson), Sulley finds a door left alone and powered. Looking inside, he finds the room empty. He quickly discovers its child, a toddler girl (Mary Gibbs), has followed him back to the monster world. Although Sully fears human contact because monsters believe that humans are toxic, she is not afraid of him, and calls him “Kitty.”
Sulley collects the girl and asks for help from his partner Mike Wazowski (Billy Crystal). Dressing the girl up as a monster, and nicknaming her “Boo”, the two later attempt to return her. They discover that Randall (Steve Buscemi), Sulley’s competitor for top scarer, is bringing children into the monster world to subject them to a device to extract their screams. Sulley and Mike, carrying Boo, attempt to alert Monsters, Inc.’s president, Henry J. Waternoose (James Coburn). However, Waternoose asks Sulley to demonstrate his scare tactics, and when Sulley growls loudly, Boo reveals herself as a human child. Waternoose, taking Boo, promised to set things right, but instead reveals he is in on Randall’s scheme, since he is worried for the future of the company, and exiles Sulley and Mike to the Himalayas in India. Sulley and Mike have a falling out. Sulley, only concerned about getting to Boo, quickly scavenges parts from stockpiles from the Abominable Snowman (John Ratzenberger), another exiled monster, and returns to the nearest village to use a door and return to Monsters, Inc. How does it end? You will have to watch it.