Cinque Izumi is a cheerful and athletic half-Japanese, half-Cornish boy who studies at Kinokawa International School in Japan, and has a normal life everyday until he suddenly got summoned to the alternate world of Flonyard (フロニャルド Furonyarudo?). The people there look no different from humans, except for one thing; they have animal ears and tails. The one who summoned him is Millhiore, the dog-like Princess of the Biscotti Republic (ビスコッティ共和国 Bisukotti Kyōwa-koku?), who requested his assistance in their war against the forces of the feline-like Galette Lion Dominion (ガレット獅子団領 Garetto Shishi Danrei?).

After responding to Millhiore's plea, he receives a sacred weapon, the Palladion (パラディオン Paradion?), and is appointed as a Hero to fight for Biscotti in a war that unlike the ones in Earth, is waged with special rules and no casualties, in a fashion similar to a sports competition with the purpose to raise funds for the Kingdoms involved, where the winner faction claims a larger sum than the losing side. After winning his first battle for Biscotti, Cinque learns that when a hero is summoned into Flonyard he becomes unable to return to his homeworld, a fact that Millhiore also was unaware of. While the scientists of Biscotti look for a way for him to return home, he decides to keep serving Millhiore as Biscotti's hero.

After sucessfully returning home, Cinque is summoned back time and again, to visit his friends in Flonyard while embarking on several adventures with them, accompained by his cousin and rival athlete Nanami Takatsuki, who is chosen to become Galette's Hero, and his close friend Rebecca Anderson who is also chosen by the First Princess of the squirrel-like Principality of Pastillage (パスティヤージュ公国 Pasutiyāju Kōkoku?) to become their hero as well...