In the world of Ars, humans lead a decades-long war against giant beasts that ravage the lands and feed on their resources. Those tasked with fighting them are champions called nagimori, who utilize the destructive power of kannagi: beings gifted with uncontrollable magical powers. While mourning the death of his war partner, former nagimori Jiiro crosses paths with Kuumi, a young woman who has just awakened her magical powers. After the pair form a contract to save the city from a giant beast attack, Jiiro learns that Kuumi has escaped a laboratory and is being hunted by the underlings of Mezami, the head of the research facility. Reluctantly agreeing to help Kuumi flee from her pursuers, Jiiro starts traveling with his new partner and her friend Myaa, a young girl with a cat headdress. As the unlikely trio wanders the Arsian lands, top officials begin setting their plans into motion—and the group of adventurers soon discover that men are to be feared more than monsters. The story takes place in an age of swords, heroes, and myths. Giant beasts created the land, but then humans stole that land. This angered the beasts, who then started eating humans. In order to fight back, humans called upon the gods. The Kyojuu beasts started spreading around the world, causing great damage, but humans fought back by hunting the Kyojuu. Humanity also prospered by to using the dissected parts of the beasts.