Fran and Serendín are brothers and live in the Gran Arrabal. They both dream of one day leaving their town and traveling to The Kingdom, a paradise on earth sheltered behind great walls.For that they will have to ride The Beast, a huge creature that merchants use to transport goods to The Kingdom. An unexpected situation complicates Fran and Serendín's plans, separating them.Sister and brother will embark on an adventure to meet again and at the same time find themselves, in a fantastic world divided by visible and invisible borders.