In the world of Ars, an ongoing conflict rages on between humans and colossal beasts that ravage the land, depleting its resources. The nagimori, skilled fighters wielding kannagi - mystical creatures with immense magical abilities, are tasked with the duty of battling these monsters. Jiiro, a former nagimori, is mourning the loss of his comrade when he crosses paths with Kuumi, a young woman who has recently discovered her own extraordinary powers. Together, they join forces to protect their city from an impending monster attack. However, Jiiro soon discovers that Kuumi has just escaped from a laboratory where she was being held captive, and the minions of Mezami, the facility's leader, are in hot pursuit of her. Despite his reluctance, Jiiro agrees to help Kuumi evade her pursuers. Joined by Myaa, Kuumi's loyal companion, the three of them embark on a journey through the vast stretches of Ars. Along the way, they realize that the machinations of the formidable authorities pose a greater menace than any monster they have ever encountered.