Mati is a nine years old know-it-all child, attracted by scientific experiments, inventions, and maps and plans. This attributes will help him to constantly rescue Rocco and find the way to make that his underground friend can stay with him.
Rocco is a restless and curious monster that comes from a remote village in the center of the Earth, and who is fascinated by the surface world. His arrival in Mati’s life will help him to beat his personal issues, creating this way a great friendship.
The old and eccentric Mr. Ramon, Mati’s neighbor and archenemy. He is a compulsive accumulator, lonely and grumpy. His mansion is a place full of things that he has collected for years.
Ramon is always in company of his pet Rufino, a really astute and gossip cat.
There is a legend witch tells that inside his object collection there is a child that one day dared to claim a toy.
Mr. Ramon is obsessed for capturing Rocco.