Sully and Mike are monsters who work for a company which specializes in providing monsters to hide in childrens' closets at night. Given a huge factory with a conveyor belt of doors soaring to the ceiling, the monsters enter the doors they're assigned and scare the children inside. Then they leave through the same door. One night Sully goes into the room of a little girl who follows him out the door into the monster factory. Sully is terrified, because he's not supposed to have anything to do with human children and knows he'd be in trouble for this slip-up. He calls the little girl Boo and becomes very attached to her. Only Mike is in on the secret. They go through a series of close encounters and foiling the schemes of an evil monster, trying to return Boo through her own door.