The energy of the city of Mostropolis is fueled by the cries and screams of the children of human beings, for this reason the monsters have invented time gaps that lead to their bedrooms, so that they can scare them and make them cry. One of the best professionals of children's scares is Sullivan, a huge blue and furry monster, always helped by Mike a little green monster with a big eye. Among its rivals we find the gecko Randall, able to blend in and make itself invisible. Monsters are also terrified of humans, particularly with regard to physical contact, as they deem them infected and any monster that comes into contact with them must be properly disinfected. One day Sullivan forgets an open passage, which allows a little girl, called Boo, to access the world of monsters. Sullivan tries in every way to get her back into his world, but fails because the child seems amused by the encounter with the monster. When he manages to imprison her in a suitcase and send her home through the passage, Randall arrives always ready to put in a bad light every misstep of Sullivan, who thus gives up his intent. Sullivan asks Mike for help, but in the meantime the monster population has spotted the little girl wandering around the rooms of a restaurant and this sows such a havoc that specialized decontamination agents intervene. Sullivan and Mike with great fear, have to hide little Boo in their home, however they will realize that the child is not dangerous or infected, but for the avoidance of doubt they disguise her as a monster, to send her back home once the gate is reopened. Will they succeed without being discovered by the evil Randall?
The DVD contains the following extra contents: - behind the scenes (making of) - short film - interactive game - documentary