A thirty-year-old man who longed to use magic dies unexpectedly and is reincarnated as a child named Shion in a new world, only to discover that magic does not exist. Disheartened and lethargic, he nearly gives up until a mysterious luminous phenomenon appears at a lake with his sister Marie, revealing that supernatural forces might be possible. Determined, Shion vows to create magic himself from scratch.

The story follows Shion as he secretly researches the nature of this power, which he calls the demonic possession state, a glow triggered by intense emotions that only certain people can see. Living with his family in a rural noble household, Shion practices alongside Marie and their friend Rose, discovering that strong feelings like joy or affection can generate a magical aura. As he experiments, he develops basic spells such as flare, bolt, aqua, and wind, pushing forward despite the physical limits of his magical reserves.

The plot advances through key conflicts, beginning when goblins attack their village, forcing Shion to test his fledgling magic in real danger. After proving its potential, he gains his father Gawain’s support and dives into full-scale magical research. However, when Marie suddenly collapses from a lethargy sickness that only magic can cure, Shion faces his greatest challenge yet. He partners with new allies including Cole, Rafina, and Brigitte to find a cure while confronting supernatural enemies like the Wraith that threaten the kingdom.

As Shion unravels the secrets of magic, he learns that his own lineage is tied to an ancient race of half-human, half-demon mages, giving him unique abilities but also placing him at the center of a larger destiny. Through monster subjugation, political alliances, and the emotional weight of saving his sister, he grows from a reclusive bookworm into a pioneer who introduces magic to a world without it, ultimately becoming a hero who changes the fate of everyone around him.