After the death of his grandfather, 19-year-old Kazuya Souma—an aspiring civil servant—is left all alone with no one to call family. Out of the blue, he is transported to the Elfrieden Kingdom, a small ailing country in another world, to be a "hero." An ongoing war with the demon army has put the entire world in peril, and Kazuya was summoned to aid in the conflict as an offering from Elfrieden to its allies. Dissatisfied with being used as tribute, Kazuya decides to help the kingdom revamp its declining economy—not by way of adventuring or war, but through administrative reform. Abruptly declared the King of Elfrieden and betrothed to the princess, the "Realist Hero" Kazuya sets out to assemble a group of talented citizens who will assist him in his bureaucratic battles to get the kingdom back on its feet.

Together with his talented aides, the "Realist Hero" Kazuya Souma continues his quest of reinvigorating the Elfrieden Kingdom through administrative reform. Having successfully conquered Van—the capital city of the Principality of Amidonia—Kazuya now faces the envoy from Gran Chaos Empire, who wishes to impose punishment for breaching the ban on war established by the Mankind Declaration Treaty. Despite the dire situation, Kazuya sees a path to avoid unnecessary conflict whilst gaining new allies. With his plan being seemingly perfect, the only barrier to success is gaining approval from the envoy.