A man who loved city-building games in his previous life is reincarnated as a baby named Ars in a troubled fantasy world where the royal familys power is in steep decline. He retains all his memories from Japan, but his new circumstances are harsh; he is born as the son of a poor farming family in a remote, cold village, struggling daily against hunger.

Witnessing this harsh reality, the young boy decides he cannot rely on a slow life. Using his wits and a natural talent for earth magic, he begins experimenting to improve his familys fortunes. He starts with small, practical projects like selective breeding of crops to increase the harvest and using magic to build simple houses. His breakthrough arrives when he successfully creates a new spell for making durable bricks. This innovation becomes his ticket out of poverty. He sells his bricks to a traveling peddler and uses the proceeds to acquire a rare egg, which hatches into a valuable tamed beast, a companion that aids him in his work.

Word of Ars abilities spreads. He is not just a farmer but an innovator, applying systematic thinking to clear land, cultivate fields, build Roman-style roads, and construct fortifications. The story follows his long-term project to build a castle brick by brick, transforming his desolate village into a thriving settlement. His reputation grows from a clever child to a figure of near-mythic status, later being called a Living God by those who witness his accomplishments.

The narrative expands from simple survival and farming to encompass regional politics and kingdom-building. As Ars grows, the decaying kingdom around him becomes more volatile. Powerful nobles vie for control, and the royal family, stripped of its former power, struggles to maintain legitimacy. After a conflict among the hegemonic noble houses leads to a power vacuum, the king himself is forced to flee from a leadership struggle and life-threatening danger. He escapes to the far north, arriving in Ars territory, the Fontana domain, seeking refuge. This event draws Ars directly into the heart of the kingdoms political turmoil. No longer just a poor farmer, he must leverage his growing resources and influence to navigate alliances, protect his people, and decide the fate of the king, setting the stage for the founding of a new nation.