Eiji Hino is a wanderer who has no place to call home and a tragic past. When metallic creatures known as the Greeed awaken after their 800-year slumber to attack humans and feed off of their desires, the disembodied arm of the Greeed named Ankh gives Eiji a belt and three Medals to fight the other Greeed as Kamen Rider OOO. The mysterious Kougami Foundation approaches Eiji and begins assisting him in his fight against the Greeed, though their true motives are not clear. As Eiji fights the Greeed and their Yummy monsters, learning more about the Greeed and Ankh, he starts to find a purpose beyond his journey.