|Eiji Hino is an individual with no family, job or dreams. When animal-like monsters called Greeed awaken after their 800-year slumber to attack humans and feed off of their desires, a disembodied arm of a Greeed called Ankh gives Eiji a belt and three Medals to fight the Greeeds. This allows Eiji to become Kamen Rider OOO. The mysterious Kougami Foundation approaches him and begins assisting him in his fight against the Greeed, but their true motives are not clear. Kamen Rider OOO has multiple combinations of powers using the Core Medals. When Eiji fights for the first time, he starts to realize what it means "to be alive". Ankh possesses the body of the detective Shingo Izumi who was mortally wounded in an attack, intending to use the body as a temporary vessel, keeping Shingo barely alive, so he could move about unnoticed while feeding on as many Cell Medals as he can to regain his complete form. Ankh underestimates Eiji's strength of will and reluctantly cooperaties with Eiji to save human lives in exchange that Eiji continue gathering Cell Medals as Kamen Rider OOO.
Later, Akira Date joins the Kougami Foundation and becomes Kamen Rider Birth, vowing to himself he will taking all the Cell Medals from here on out by gathering them into a milk tank he carries around when not fighting. When he first sees Eiji's picture in the Cous Coussier, he says that he may have seen him before he returned to Japan. Date is training Shintaro Goto to take up the mantle of Kamen Rider Birth once his job with the Kougami Foundation is done.