Arthur (Freddie Highmore) lives with his widowed Granny (Mia Farrow) in her quiet little house in the country. The house is threatened with foreclosure and Granny with eviction in two days if the nasty developer Davido (Adam Le Fevre) isn’t paid a huge sum of money. Arthur is only 10 years old but he knows it is up to him to save Granny’s home. She has often entertained him with tales of the Minimoys, a people so small they are all but invisible, and the cache of African rubies they hold. Arthur finds clues left by his grandfather Archibald (Ron Crawford) that lead him to discover the way into the tiny world of the Minimoys. Once there he discovers their underground city is under threat of flooding by the mosquito-borne troops of the rebel known as the Evil M (Alain Bashung). Arthur teams up with Princess Selenia (Mylène Farmer) and her younger brother Bétamèche (Cartman) to save the Minimoys. But he soon discovers that not only does the Evil M possess the African rubies he also holds Arthur’s grandfather as a prisoner in his city of Necropolis.