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.

Located at the end of the world, the Kingdom of Ozmargo is inhabited by beasts of all shapes and forms, ruled by a fearsome nameless king. After a hundred-year war between Ozmargo and the neighboring human kingdom ended, a peace treaty was signed: every year, a sacrifice must be sent to be devoured by the king on the night of revelation. Sariphi is the 99th and latest human sacrifice. Although frail-looking, she neither fears the king nor has a family or home to return to should she manage to escape from her impending demise. Intrigued by the audacious girl, the king lets Sariphi roam around the palace despite the disapproval of his advisor, Anubis. Little by little, the king and Sariphi begin to warm up to each other, but the night of revelation is fast approaching. Unbeknownst to them, the fateful day unleashes a series of events that will change the course of destiny of not only Sariphi and the king but also of their respective worlds.

The Gates of Hell reopen, throwing the world into chaos and setting demons free to terrorize the land once again. These troubles seem irrelevant to quirky villager Touka Scott—until a demon attacks Yuna Eunice, his childhood friend. Luckily, the legendary hero Sion Blandan arrives in the nick of time to strike down the vile creature. Unfortunately, Sion soon dies after accidentally falling into a trap Touka set to defend his radish farm. Anri Haysworth, Sion's necromancer companion, resurrects the hero by transferring Touka's soul into Sion's corpse. Now possessing the body of the legendary hero, Touka is tasked with resealing the Gates of Hell and defeating the demons that threaten peace. Together with Yuna and Anri, Touka copes with both his new reality and Sion's heroic legacy as he embarks to save the world.