Fran and his sister Serendin live in the Great Slum and dream to one day travel to The Kingdom, a paradise on earth sheltered behind huge walls. In order to get there, they need  to do something many have tried but just a few have succeeded: to smuggle on the Beast, a legendary creature that merchants use to transport goods to The Kingdom. When they finally decide to do it, an accident happens and Fran get separated from Serendín. Brother and sister will embark on an adventure to find each other and at the same time find themselves.