A world where the human and demon realms exist as separate domains. The demon realm was once a world of absolute power where the strong ruled over the weak. Numerous demon lords fought to claim their own territories, but the Demon King Guest subjugated all factions, uniting them into a single great nation for the first time. Currently, Guest rules the demon realm from the Demon King's Castle located at its center. Due to their overwhelming power, Guest is regarded as an absolute being by demons and is famous enough in the human realm to be a target of extreme caution by name alone. And by Guest's side is the Demon Queen, Serena Veloa. However, the actual daily life in the Demon King's Castle is quite different from what people imagine. While Guest is receiving important reports on the throne, Serena might appear with snacks and cling to their side, or stare intently at Guest during a meeting, claiming she's bored. The subordinates of the castle are already accustomed to this sight. Despite her high status as the Demon Queen, Serena does not flaunt her power and mingles comfortably with the demons. Thanks to this, the image of the terrifying Demon King Guest and the beloved, cute Demon Queen Serena has become firmly established in the castle. The story focuses on the daily life of the royal couple in the castle and the incidents that arise while ruling the demon realm, rather than grand wars. While major events like rebellions or human invasions occasionally occur, the story usually follows the interactions between the castle's subordinates and various demons.