Monsters, Inc. is the biggest scare factory which exists; it is directed by Mr. Waterloose. Located in a parallel dimension, the factory builds portals that transport the monsters for children's bedrooms, where they scare the kids and generate the energy source necessary for the survival of the factory and the monster’s city. Among all the monsters who work there the most frightening is James P. Sullivan, a huge, intimidating monster with blue spots and horns, which is called Sully by his friends. His assistant is Mike Wzowski, a small oval monster with one eye whose mission is to assist in Sully’s job: he prepares, trains and motivates Sully before he enters the child's room, after the fright, he checks if Sully touched or get close to an object of the child's room because they are considered toxic by the monsters who believe that contact with the kids would be catastrophic to monster’s world. However, when they were visiting the human world to work, Mike and Sully met the girl Boo, who has just inadvertently ending up in the world of monsters and causes the expulsion of both for the real world.
When Sully and Mike discover that children are not toxic and that Randy was cheating to be the "employee of the month," Sully decides to become director of Monsters, Inc. and he modifies the enterprise system. Mike let Celia very proud of him because he helped to unmask Randy and Mr. Waterloose. Mike asks Celia to marry him. Waterloose becomes Celia’s assistant, she ceases to be a receptionist and becomes aide to Sully, in the direction of the company.
Mike becomes coordinator of the monsters that make performances for children. Randy after being discovered by Mike and Sully becomes "experimenter" doors; his function is to test new doors that do not have a certain destination. When Randy will test a navy blue door, the system fail, and he freaked out because he was "inside" the door, damaging it with kicks, for this reason it stuck in an unfamiliar place, and can not be found.
Mike fix the Boo’s door and Sully can talk to the girl's parents to ask permission for them to the girl woked beside him. After three attempts to talk to the girl's family, Sully finally managed to keep the conversation without Boo's mother fainted. The Sully’s idea for the girl is that she will be a manager in the new structure of the company, making sure that the monsters are doing the funniest performances for children, because their laughter is much more powerful than the crying, she is also responsible the selection of monsters that will perform for children, their admission test is very simple: if she laughs until her stomach hurts the monster is accepted.
In October, the company has a drop in production, because it is the month of Halloween the new kids in the monster’s business do not always believe it is a true monster, often thinking that is a costume, which makes the laughter produced can not be used. To avoid a bigger problem, Boo still has the bright idea of allowing children to pass through the world of monsters so that they discover the reality of monsters, realize that they are real, thus creating emotional link between children and monsters, and suggests that Mike create a park for monster’s children human’s children, which leverages the "production" of laughter to the city.
Later in that year, Sully received the "honorable monster" award, for his improvements in the monster world, and he dedicated the award to his friend Mike and Boo.
