The protagonist, a Japanese high school student, is suddenly transported to a medieval-European-esque world while walking to the hospital to visit his father. He finds himself in a grand hall, surrounded by people with deep-set features, dressed in ornate robes and cloaks. A stout man, clearly a king, proclaims him a hero summoned to this world. The protagonist experiences a painful surge of knowledge and power, understanding the king's words and the nature of his situation. He learns that upon being summoned, heroes gain the necessary knowledge and power to survive in this world. He also discovers that a golden bracelet on a pedestal is a "bracelet of subjugation," a magical item used to control its wearer.
The protagonist, now named Jin, questions the king about returning home. The king promises he can return after defeating the Demon Lord, but Jin senses deceit in his eyes. Feigning a headache, Jin requests rest and is led to a luxurious room at the top of a spire, which he recognizes as a prison. Using his newly acquired knowledge and the "Dimensional Storage" ability, he gathers items from the room and escapes by leaping from the window to the castle walls, alerting guards and fleeing into the capital city.
Five years pass. Jin lives in the multi-ethnic nation of Almoran under the name "Jin," blending in due to its diverse population. He works various jobs thanks to his hero powers and continues to research a way back home without success. He befriends Daluk, a lizardman who despises the Demon Lord's territory due to the persecution of non-demon races. Daluk informs Jin about the discovery of an ancient ruin dungeon, rumored to contain a sealed "demon god" capable of granting any wish. Hopeful, Jin prepares to explore the dungeon, located beneath a royal tomb.
Inside the tomb, Jin encounters a Basilisk and a group of adventurers. He intervenes, saving the adventurers and healing one of them, Ash, who had been petrified. However, the other adventurers attempt to ambush him for his loot. Jin swiftly dispatches them and collects the Basilisk's materials. He proceeds deeper into the dungeon, facing numerous powerful monsters. He eventually finds a treasure room, but no demon god. A voice leads him to a hidden chamber where he meets Aina, a beautiful woman bound by chains, who reveals herself as the wish-granting demon god.
Aina explains she can grant unlimited wishes in exchange for a master-servant contract, requiring only meals. She was sealed by her previous master, a king, who feared her power. Jin, wary of being deceived, asks Aina to reveal her true form, which turns out to be a young girl. He breaks her chains and asks to be returned to his world. Aina reveals that returning to his "higher dimension" world is impossible with her current power, as it requires immense energy. Disheartened, Jin learns that Aina can grant him "half the world" by having him defeat the Demon Lord and usurp his throne.
Aina teleports Jin to the Demon Lord's castle. He confronts the Demon Lord, Satanabulus, and, with Aina temporarily sealing the Core Crystal's power, defeats him. Aina then broadcasts Jin's victory, declaring him the ruler of half the world. The demon race, valuing strength, accepts Jin as their new king, the "Hero King."
Jin struggles with his new role, finding the political machinations and expectations overwhelming. He learns about the country's dire financial situation and the corruption of the former Prime Minister, Lopez. Jin dismisses Lopez, rewards loyal subordinates, and begins implementing reforms, including establishing salt fields and improving the agricultural system. He also encounters various factions and individuals, including the loyal beastkin Shibu, the pragmatic demoness Lowrie, the stoic dark elf Saeria, and the seemingly subservient but potentially manipulative frogman Lopez.
Jin's days are filled with administrative duties, dealing with political intrigue, and trying to understand the complex social dynamics of his new kingdom. He also continues to explore his powers and the world around him, often with the assistance of Aina and his growing circle of allies. He learns more about the world's history, its magic system, and the various races and their customs. Despite the challenges, Jin is determined to create a better future for his new kingdom, all while still harboring the hope of one day returning to his original world.
