We never knew that in such a world lived such a group of monsters. They look comical but must pretend to be ferocious. Because, in the dead of night, they quietly appear in children's closets to frighten babies who have just fallen sweetly asleep. All of this is simply because they are employees of Monsters, Inc., and children's screams are the sole source of power for the entire monster kingdom. The monsters are afraid of children, yet they must make them scream. In the land of monsters, they are all heroes who benefit the entire populace. Sulley (John Goodman) is the most outstanding employee at Monsters, Inc., always leading in performance, having scared countless children to tears. He and his partner Mike Wazowski (Billy Crystal) are highly respected by everyone. By chance, Sulley accidentally brings a two-year-old girl named Boo (Mary Gibbs) back to the monster world, causing immense panic. The mischievous and adorable Boo turns these terrifying monsters upside down with fear. The authorities are in pursuit of Boo, while the villainous Henry J. Waternoose (James Coburn) also plans to use Boo in his evil scheme to reform the factory. On the other hand, as Sulley spends time with Boo, he gradually develops an inexplicable affection for her, even to the point of being exiled to the wilderness because of it. Yet nothing can hinder Sulley's determination to save the girl. For Boo's sake, Sulley embarks on a perilous journey...