In 1960, 10-year-old protagonist Arthur is living with his grandmother in a quiet country house. His grandfather Archibald has recently gone missing and he sees little of his parents. His grandmother entertains him by reading stories to him of his grandfather's adventures in Africa. One of these stories is about the friendship of the tall, Masai-like tribe called the Bogomatassalai with the Minimoys tooth-sized, elfin beings they believe to be their counterparts. After Archibald had helped the Bogomatassalai, says the story, they and the Minimoys had given him a large amount of rubies in a show of thanks for helping them. After retuning home, Archibald brought the Minimoys with him and lent them his garden as their home. According to the story, he gave the Minimoys the rubies to look after them until he needed them. Arthur has become enamoured of a picture of Selenia, the princess of the Minimoys. It is later shown that Arthur's grandmother has been given a two-day deadline to pay a large sum of money to a building developer named Davido, who has bought her mortgage from the bank and plans to evict the two. Arthur therefore decides to find the rubies in order to pay off the debt. After stumbling across various clues written by his grandfather, Arthur finds the key to the Minimoys' world. He is met in the garden by the "Bogomatassalai", who show Arthur how to get into the Minimoys' world by shrinking down to their size and becoming a Minimoy himself. This is done by means of a telescope and the power of a shining full moon. When Arthur is underground with the Minimoys, he learns of a danger to the little people's future, posed by the plans of Maltazard ("Malthazar" in the movie), the leader of an army of mosquito-riding rebels named 'Seides' living in nearby "Necropolis", who plan to conquer the Minimoys by flooding their city. Maltazard is said to have been a Minimoy war hero corrupted by a seductive weevil, by whom he has a son called Darkos. After his corruption, Maltazard became a warlord known as the "Evil M", keeping Arrchibald as his prisoner and the rubies as his throne. Arthur, reflecting his legendary British namesake, draws a sacred sword from its recess, which he then uses to protect his new friends from Malthazar's soldiers. The king gives permission for Arthur to travel to Necropolis, sending with him the princess Selenia and her younger brother. Armed with Arthur's new sword the use of which they share the trio embark. En route to Necropolis, they are numerous times put in danger by Maltazard's soldiers, who are making plans for the coming flood. During their travel Selenia reveals that at her age (which corresponds to Arthur's), she must find a husband within two days. Over the course of the journey, Arthur comes to fall in love with Selenia, a fact she sees and uses to her advantage. Eventually, the questors arrive in Necropolis, where the stubbornly independent Selenia takes the sword and leaves the others. Before doing so, she kisses Arthur, marks Arthur as her husband and the future king. Selenia then confronts Maltazard alone. Maltazard wishes the first kiss of the princess to restore his original beauty; after learning she has already kissed Arthur and she can no longer give him her powers, he decides to kill her. She is captured and incarcerated in a cell where she finds her companions, who have also been seized. The kiss scene was cut in the U.S. version, and the order of the scenes was changed. While in prison, they find Arthur's grandfather, who like Arthur has become a Minimoy. This transformation has altered the proportions of his body, giving him a larger head, thinner limbs, long ears, and broad extremities. He has been teaching his knowledge to Maltazard in exchange for food. Selenia feels defeated, but Arthur comforts her. Maltazard allows them to escape 3 minutes ahead of the flood, which they do by using a clockwork car that Arthur possessed, in his human form, until Davido kicked it down the drain. During the adventure, Arthur's crush on Selenia has been supplemented by comradeship, as well as by her own affection for him. When Arthur learns of a Minimoy custom stating that newlyweds should not kiss for the second time until after ten moons, Selenia decides she does not wish to wait and gives Arthur a long, passionate kiss before he returns to his human form. He now has an hour wherein to save the Minimoys from the flood. Archibald returns with him to the human world and is reunited with Arthur's grandmother. With the help of a royal advisor's long-lost son, Miro, Arthur, in human form, is able to prevent the flood from destroying the Minimoys, redirecting it to Necropolis instead by dropping a ball into Necropolis, smashing the drinking straws used to direct the water and blocking the tunnel it was heading through, flooding Necropolis. Maltazard abandons the plan, his army, and his son, and flees. The force of the water ejects the rubies and Miro. Arthur and Archibald pay Davido with one ruby; he tries to take all, but the Bogomatassalai capture him and give him to the authorities (scene cut in the U.S. edition). The story closes with the vows spoken by Arthur in his human form to Selenia, who sits in a tree, to wait patiently until the time is right for them to be together. The ending credits then proceed, alongside scenes of every significant character (as well as the director, Luc Besson) appearing in Minimoy form to bow.