Eugene Santafield has a dream of becoming a magic swordsman, but he faces a seemingly insurmountable problem: he cannot use any form of attack magic. Instead, he is blessed with extraordinarily powerful defensive and healing magic, an irony that makes him a target of ridicule from his peers. Determined to escape his past and the people who know of his failures, Eugene flees to the Lykeion Magic Academy.

It is beneath this very school that his fate takes a turn. He meets Erinyes, a beautiful and powerful Demon Queen who once ruled the world but is now imprisoned underground. Recognizing the untapped potential in Eugene's unique abilities, Erinyes appoints him as her caretaker. Not long after, the headmaster of the academy assigns Eugene a second, equally formidable task: to serve as the guardian of Sumire, a mysterious girl from another world who is, in fact, an Ifrit of legend.

The story follows this unlikely trio as Eugene navigates his new life. Despite being labeled a failure, his "useless" defensive magic proves to be the perfect complement to the overwhelming power of his two companions. His relationship with the fallen Demon Queen Erinyes is particularly intimate and complex, as she offers him pragmatic advice, a telepathic link, and a physical relationship, all while her true motivations remain ambiguous. Meanwhile, Sumire, who is shy and empathetic, seeks not only to recover her lost memories but also to become Eugene's partner on and off the battlefield.

The central conflict escalates as Eugene forms a party with Sumire and later with Sara, the student council president who becomes a rival for his affections, to explore the Last Dungeon. Within this dungeon, he discovers that by using Sumire's powerful Ifrit mana, he can finally bypass his zero attack power and deal damage to enemies, rekindling his abandoned dream. However, a nasty surprise awaits him at Level 100. As he delves deeper, the narrative explores his prestigious lineage, his complicated feelings for Erinyes, and the growing harem of women—including his former partner, Sara—who compete for his attention. The story balances dungeon-crawling action with comedic romantic tension, as Eugene proves to be socially aware and uncharacteristically honest, having already admitted his feelings for the demon queen.