Justo cuando su abuela está a punto de perder la casa en la que vive, Arthur, un niño de 10 años, recuerda que su abuelo le había hablado de un gran tesoro oculto en la tierra de los Minimoys, un universo de pequeños seres fantásticos que viven en armonía con la naturaleza, un mundo tan lejano y sin embargo tan cercano e invisible al ojo humano. Decidido a salvar a su abuela, Arthur conseguirá la llave para entrar en él y allí conocerá a la princesa Selenia y a su hermano Betameche. Juntos buscarán el tesoro escondido y deberán enfrentarse al diabólico M, en una aventura que nos enseñará que, en ocasiones, los pequeños héroes son capaces de conseguir grandes logros.

Arthur is a spirited ten-year old whose parents are away looking for work, whose eccentric grandfather has been missing for several years, and who lives with his grandmother in a country house that, in two days, will be repossessed, torn down, and turned into a block of flats unless Arthur's grandfather returns to sign some papers and pay off the family debt. Arthur discovers that the key to success lies in his own descent into the land of the Minimoys, creatures no larger than a tooth, whom his grandfather helped relocate to their garden. Somewhere among them is hidden a pile of rubies, too. Can Arthur be of stout heart and save the day? Romance beckons as well, and a villain lurks.