A middle-aged Japanese office worker is accidentally caught in a hero-summoning spell and transported to the fantasy world of Mildgard. His body is disintegrated by the spell, but the goddess Nolyn, apologizing for the error, resurrects him as a seventeen-year-old boy named Takumi Iruma. As further compensation, she grants him a single special skill. Hoping for a quiet, peaceful life away from combat, he chooses a seemingly mundane creation ability: alchemy.

Takumi soon discovers that his chosen skill is anything but ordinary. Alchemy proves to be an incredibly versatile cheat ability that allows him to create almost anything he can imagine, from holy swords and enchanted armor to flying ships and magical tools. Despite his desire for a low-profile existence, his extraordinary creations begin to attract considerable attention. He settles in the city of Bolton, where he joins the local adventurer's guild to maintain his employment as an alchemist, completing at least one quest per month. His inventions and growing reputation eventually draw the notice of the powerful Holy Sidonia Empire, which, inspired by Takumi's abilities, summons three heroes from his original world.

As Takumi builds his new life, he is joined by a growing group of companions. His first partner is Sofia Sylphid, an elf swordswoman who was crippled and cursed before Takumi heals her, leading her to become a loyal and caring adventurer. Maria becomes his cheerful housekeeper and Sofia's best friend, skilled in both domestic work and combat. He tames a Killer Spider he names Maple, who produces valuable silk and later evolves into an Arachne capable of speech. He also purchases Reyva, a fox-girl slave with magical talent and an interest in alchemy, and her friend Máni. Together, this found family navigates the opportunities and dangers of Mildgard.

Key narrative arcs follow Takumi as he establishes his alchemy business, develops revolutionary new products, and faces threats from those who covet his power. The simultaneous story of the three heroes summoned by the Sidonia Empire runs parallel to his own, with their paths eventually crossing. Takumi must balance his longing for a peaceful, productive life as a craftsman with the reality that his abilities make him a central figure in the world's politics and conflicts, forcing him to question whether he can truly remain on the sidelines.