A 30-year-old man whose greatest dream was to wield magic dies unexpectedly and is reborn into a fantastical new world. Reincarnated as a young boy named Shion, he grows up as the shy and bookish younger child of the Ornstein family, lower-ranking nobles who govern a rural territory in the kingdom of Ristia. He is lovingly cared for by his energetic older sister Marie, who adores him and spends her days training in swordsmanship with their father. Shion initially believes his fantasy has come true, expecting this new world to be filled with the sorcery he always longed for. His hopes are brutally crushed when he learns that within this realm, the very concept of magic does not exist. The word itself is unknown, and the phenomena he describes—summoning fire, water, or light from nothing—are dismissed as impossible.

Falling into a deep depression that lasts for years, Shion resigns himself to a life without the one thing he cherished most. However, a mysterious event reignites his spirit. One day, Marie brings her despondent brother to a secluded pond in the forest, where they witness a strange and luminous phenomenon that defies all physical explanation. This is the proof Shion needs that some form of supernatural energy does exist. With this discovery, he makes a solemn vow: if magic does not exist in this world, he will create it himself.

Shion begins a dedicated and secretive research project with Marie as his sole confidant. He discovers that powerful, positive emotions like joy and affection can generate a palpable energy he calls the demonic possession state, the foundation of his new magic system. Through painstaking trial and error, he learns to manipulate this inner mana. His initial successes come in the form of a protective aura coating his body, and he soon develops the first true spells: Flare, a small but dangerous flame; Bolt, a discharge of electricity; and Aqua, a stream of water. As his research deepens, he pioneers complex magic by combining multiple elemental spells and even crafts specialized equipment, like conductive gloves, to better channel his power.

As Shion grows from a melancholic child into a determined ten-year-old researcher, he and Marie are joined by their childhood friend Rose in monster-hunting quests to test their abilities. Their peaceful research is shattered by tragedy when Marie suddenly collapses, struck down by a mysterious and devastating affliction known as lethargy sickness. She becomes a hollow shell of herself, unable to move or respond. Desperate to save the sister who has been his unwavering support, Shion throws himself into his greatest challenge yet. He assembles a team with new allies, including a boy named Cole and noble girls Raphina and Brigitte, to research a magical cure for the incurable disease. In the process, his mission expands from personal salvation to a larger destiny, as the spread of this magic-requiring sickness and the emergence of supernatural monsters that can only be defeated by sorcery position Shion to become the pioneer who introduces magic to the entire world.