Summoned to another world together with his classmates, Akimitsu Tsukasa ends up wandering alone in this unfamiliar land after being betrayed by the friend he thought was his best ally. Drifting into Eigiiyu, a settlement where the horned race known as “demons” live, Tsukasa survives day by day as a “free mercenary,” relying solely on the gift he obtained during the summoning — the outdated and forgotten magic known as Seal Arts. One day, his childhood friend Matsuzaka Mihono suddenly visits him with a desperate request: “Please… save my little sister, Shihono.” The condition Tsukasa offers in exchange for helping her is—?