Eugene Santafield was born with a cruel irony. As the son of a famous swordsman and a promising student who once mastered the blade, his dream of becoming a magic swordsman was shattered when his magical aptitude manifested. He was blessed with extraordinarily powerful defensive and healing magic known as white mana but possessed absolutely zero talent for offensive magic. In a world where swordsmanship is meant to be combined with attack magic to be effective, he is ridiculed as incompetent by his peers. His childhood friend and fiancée, Princess Airi, abandons him, and he is cast aside from his former life.

Determined to escape those who know of his failures, Eugene flees to the Lykeion Magic Academy. There, he discovers a fallen Demon Queen named Erinyes, who once ruled the world, imprisoned beneath the school. Recognising the unusual potential in his overwhelming defensive capabilities, she appoints him as her caretaker. What others see as useless becomes essential, and he is assigned the task of looking after the beautiful but confined Demon King.

Soon after, the academy headmaster gives Eugene another assignment. He must become the guardian of Sumire, a mysterious girl from another world who manifests as an Ifrit, a legendary being of immense fire power. With Sumire, Eugene discovers a new possibility. Her Ifrit mana is so powerful that even his white mana cannot overwrite it, and through her, he can finally channel power that allows him to deal damage to his enemies while remaining protected.

Eugene forms a party with Sumire and enters the Last Dungeon to search for her lost memories. Along the way, the student council president Sara, who was once his partner, becomes a rival for his affections, jealous of his growing closeness with Sumire. Eugene continues to pursue his abandoned dream of becoming a magic swordsman, now able to fight effectively, while navigating a complicated harem dynamic. Erinyes can occasionally assist him with her dark mana from her prison, providing a deus ex machina in critical battles.

The story explores the balance of power as Eugene climbs the dungeon floors, with a nasty surprise waiting at Level 100. His lineage as a man of destiny is revealed, and he learns that working hard with his unique white magic has its own advantages. When Eugene returns to his home city to visit his family, he reunites with his ex-fiancée Airi, who still loves him. He also makes a startling discovery about his origin at a church, explaining why he possesses white healing magic. A legendary sealed beast becomes unsealed in the kingdom, and Eugene must use his powers to save the world, strengthening his bonds with his lovers in the process.