In a modern-day Japan transformed by the sudden appearance of magic and dungeons, society has been forced to adapt to this new fantasy reality. The story follows Gurimu Sakaki, a young man born with the highest S-rank talent in life magic, a branch of sorcery widely dismissed as useless for the practical tasks of dungeon exploration and monster subjugation. Because of this, he is looked down upon by his peers and assigned the role of a临时 teacher and caretaker at the Jibik Magic Academy, treated more as a handy janitor than a respected adventurer.

Despite the ridicule and his own quiet demeanor, Gurimu possesses a deep-seated determination to prove that his seemingly mundane abilities have untapped potential. His fortunes change during a dungeon expedition when he acquires the rare and legendary Sage System, a unique item that grants its user the power to create and develop entirely new magical spells. Armed with this system, he begins a journey of rigorous experimentation and systematic testing, exploring the hidden combat and support applications of life magic. Through clever thinking and scientific-like analysis, he discovers that what is labeled as "life magic" can manipulate physical forces and materials in ways that defy conventional combat logic, turning a perceived weakness into a revolutionary new fighting style.

The narrative centers on Gurimu's quiet but relentless rise from an undervalued school custodian to an innovative force in the adventuring world. As he develops his unique abilities, he also becomes a mentor to a group of students, and his unconventional teaching methods and growing power begin to challenge the established hierarchy of the magic academy and the wider adventurer society. The story arcs focus on his dungeon conquests, his vindication against those who mocked him, and the systematic dismantling of the prejudice surrounding life magic, ultimately proving that no ability is truly worthless when wielded with creativity and persistence.