We don't know that there are such a group of monsters living in such a world. They look funny, but they have to pretend to be ferocious. Because, in the middle of the night, they have to quietly appear in the children's closet and scare the babies who have just fallen asleep sweetly. All this is just because they are employees of Monster Power Company, and the screams of children are all the energy for the power generation of the Monster Kingdom. The monsters are afraid of children, but they must make children scream. In the monster kingdom, they are all heroes who benefit the whole people. Sulleyvan (John Goodman) is the most outstanding employee of Monster Company. His performance is always leading, and he has scared countless children. He and his partner Mike (Billy Crystal) are very popular with everyone. By chance, Sulley accidentally brought a two-year-old girl Abu (Mary Gibbs) back to the monster world, causing great panic. The naughty and cute Abu scared these terrible monsters. The police are chasing Abu, and the villain Henry (James Cobb) also plans to use Abu to carry out his evil plan to reform the factory. On the other hand, in the process of getting along with Abu, Maoguai gradually developed an inexplicable affection for her, and was even exiled to the wild because of this. However, nothing can stop Maoguai's belief in saving the girl. For Abu, Maoguai embarked on a dangerous journey...