Monsters live in a hidden world, away from humans whom they fear. The monsters are so different and have so diverse interests that they have some regalities. But Nessy, Yetti, Shewolf and Coco have created some sort of a family. But one night, a 4-year-old girl named Hanna discovers Coco in her room and she follows him to Monsterville. But when she arrives, they panic: if she is not returned in time, she might get trapped in Monsterville forever and humans will come looking for her. They are scared of her, but they manage to accept her differences. But when the Cyclop and Gargoyle, their rivals, kidnap the girl, they will have to put their differences aside to save Hanna from a dark plan.