Thornmail is a story about Naoto a high school student that lives in New Horizon city, A sky city where all kinds of creatures coexist. But there are creatures that represent a treat to all around them, thus they must be hunt down. Naoto and his sister use their abilities and their special training by the legendary Swordsman Masato Himura to fight these creatures, their lives go by without many problems. Until silver hair knight apears and change this lifestyle.