Humanity teeters on the brink of extinction, driven from its lands a century ago by monstrous beings called Fiends against which conventional weapons are useless. The survivors are sheltered behind a vast magical barrier known as Babel, living in seven fractured nations. The only effective defense comes from Magicmasters, humans who wield magic and are ranked numerically—the lower the number, the greater the power. Among them, sixteen-year-old Alus Reigin stands as the number one Magicmaster, having reclaimed more lost territory and slain more Fiends than anyone alive. Despite his unparalleled service, Alus wishes to retire from the military. His request is denied, and he is instead assigned to the Second Magical Institute as a student, ordered to hide his true identity and train the next generation of Magicmasters.

There he is paired with a partner, Loki Leevahl, and given two female apprentices: Tesfia Fable and Alice Tilake. He also re-encounters Felinella Socalent, the daughter of his former superior. While navigating the mundane challenges of school life and the growing entanglements with these young women, Alus continues his research and secretly eliminates Fiends that threaten humanity. The narrative unfolds through several major arcs: the early adjustment to academy life and the first missions into the Outer World, where Fiends roam; the confrontation with the mad scientist Godma Barhong and his Dolls; the Seven Nations Friendship Magical Tournament, where Alus must compete without revealing his rank; the reclamation of the frozen territory of Vanalis; and the rising threat of the criminal organization Kurama and its leader Elise, as well as the assassination group Aferka. Throughout, Alus uncovers darker conspiracies involving the noble families, hidden rulers, and a catastrophic second coming of the Fiends. Alus's mysterious past in the Special Fiend Attack Unit and the truth of its destruction becomes a pivotal revelation. Balancing his desire for a quiet retirement with the ever-growing demands of his secret role, Alus must protect those around him while preparing for the escalating war against both monsters and human schemers.