The action of the film takes place in Monstropolis, where a company is based where monsters of all shapes and sizes work. James P. Sullivan (Sully, for friends) and his good friend Mike also work for this company whose main resources are the screams of scared children. All members of this company believe that all children are toxic, any direct contact with them is prohibited. But one day, due to a mistake by Sully, a little girl named Boo follows him into the world of monsters in the company. Thus, in order not to lose her job, Sully must find a way to repair the mistake made and to send the little girl back to her world.
Fortunately, next to him is his good friend Mike who will help him. Together, they will have adventures like they never thought they would meet.