In the year 1960, protagonist 10-year-old Arthur lives with his grandmother Daisy in a quiet farm house on a dirt road, in a small rural community in Northeastern Connecticut (based on Sterling, Connecticut). His grandfather Archibald has recently gone missing and he sees little of his parents (who are away looking for work). Daisy entertains Arthur with stories of his grandfather's adventures in Africa, featuring the tall Bogo Matassalai and the minuscule Minimoys, of whom the latter now live in Archibald's garden, protecting a collection of rubies. Arthur becomes enamoured of a picture of Selenia, the princess of the Minimoys. When Daisy receives a two-day deadline to pay a large sum of money to a building developer named Ernest Davido, who plans to evict the two, Arthur looks for the rubies to pay off the debt and discovers various clues left by his grandfather. He is met in the garden by the Bogo Matassalai, who reduce Arthur to Minimoy size. From the Minimoys, Arthur learns that they are in danger from Maltazard, a Minimoy war hero who now rules the nearby 'Necropolis', after corruption by a weevil, by whom he has a son named Darkos.
Three years have passed since Arthur has saved his grandparents home and saved the minimoys from the evil M known as Maltazard, Arthur stays with his grandparents for the holidays, during which the Bogo Matassalai (a fictitious African society) assign Arthur a series of tests including camouflage and environmental nonviolence. Having passed these tests, Arthur prepares to see the Minimoys to celebrate, until his father decides to take him and his mother back to the metropolis. When a spider gives Arthur a grain of rice containing a distress-call, which he believes has come from the Minimoys, he returns to his grandparents' house, where the Bogo Matassalai's attempt to give him Minimoy stature through a telescope fails, and they instead wrap him in vines of increasing tightness until he falls in a drop of sap into the Minimoy Max's bar. ''En route'' to investigate the Minimoys' condition, Arthur and Max rescue Betameche, who leads Arthur to the King. He then learns that Selenia is held by Maltazard, who is inspired to invade the human world by increasing his own size. Maltazard tricks the Minimoys into completing this design. The telescope itself is destroyed in the process, leaving Arthur trapped at his Minimoy size.
Maltazard a réussi à se hisser parmi les hommes. Son but est clair : former une armée de séides géants pour imposer son règne à l’univers. Seul Arthur semble en mesure de le contrer… à condition qu’il parvienne à regagner sa chambre et à reprendre sa taille habituelle ! Bloqué à l’état de Minimoy, il peut évidemment compter sur l’aide de Sélénia et Bétamèche, mais aussi – surprise ! - sur le soutien de Darkos, le propre fils de Maltazard, qui semble vouloir changer de camp. A pied, à vélo, en voiture et en Harley Davidson, la petite troupe est prête à tout pour mener le combat final contre Maltazard. Allumez le feu !
Arthur est un jeune garçon de 10 ans, vif et ingénieux qui passe toutes ses vacances dans la maison de sa grand-mère adorée. Et quelles vacances ! Le jardin de sa grand-mère abrite en effet tout un monde invisible à nos yeux… Celui des MINIMOYS ! Arthur, lui, a découvert le passage qui lui permet d’explorer ce monde et de devenir à son tour un MINIMOY. Et plus encore le MINIMOY qui sauvera ce peuple minuscule de multiples menaces dont l’invasion des armées de Maltazard, l’ennemi juré des MINIMOYS, qui règne sur les SEIDES. Petit garçon solitaire et rêveur, Arthur se trouve catapulté dans la peau d’un héros téméraire qui mènera tout un peuple vers la reconquête de sa liberté, accompagné de ses camarades Sélénia et Bétamèche. Avec un tel trio, le chemin de la victoire ne sera pas sans surprise.
