A Japanese salaryman named Takumi Iruma is accidentally caught in a summoning spell meant to bring heroes to the fantasy world of Mildgard. His body is destroyed in the process, so the goddess Nolyn, as compensation, resurrects him as a seventeen-year-old and grants him any skill he desires. Wishing for a peaceful, quiet life away from conflict, he chooses what seems like a harmless crafting ability: alchemy. To his surprise, this skill proves to be incredibly powerful, allowing him to create anything from holy swords and enchanted armor to magical toilets and flying ships.

The story is set in Mildgard, a world of swords, magic, and political tension. Takumi settles in the village of Baude before moving to the city of Bolton, where he registers with the Adventurer's Guild and begins crafting goods. Among his first creations is a magical purification device for toilets, which becomes a commercial success and attracts the attention of powerful figures, including the neighboring Holy Empire of Sidonia. This unwanted attention disrupts his hopes for a simple life, forcing him to defend himself and his growing circle of allies.

To assist with his work and protection, Takumi acquires two former slaves: Maria, a cheerful and capable housekeeper and fighter, and Sofia, a cursed elf warrior whom Takumi heals. He also tames a Killer Spider he names Maple, which evolves into an Arachne and becomes a loyal companion. Together, they form the adventuring party Wings of Nolyn, later joined by a fox-eared alchemy enthusiast named Reyva and a bunny-girl widow named Máni. The group takes on quests, explores dungeons, and develops new inventions, all while the Holy Empire of Sidonia grows more hostile.

The narrative progresses through several major arcs. The early chapters focus on Takumi establishing himself in Bolton, forming his party, and dealing with local threats like a Goblin General. The empire soon dispatches agents and eventually summons three heroes from Takumi's own world to capture him. After surviving an ambush, Takumi is tasked by the Marquis Godwin with building a pioneer village called Wedgefort in monster-infested southern lands. He successfully establishes the settlement, which becomes a refuge for those fleeing the empire. The story escalates when the Sidonia-empowered heroes massacre innocents in a neighboring kingdom, and Takumi defends Wedgefort from a massive monster horde, personally defeating a two-headed dragon. He also confronts and humiliates the corrupt heroes, leading one of them, Akane, to defect and join his side.