Deep in the mountains, an isolated village lives in fear of the dragon’s roar. When that roar comes, it is not a beast but helicopters descending, and the armed soldiers who pour out are not there to trade. They slaughter the village adults in search of a young hunter named Yuru. Yuru has spent his life hunting game and fighting off bandits, but he is unprepared for this. He races to save his twin sister Asa, who has been kept locked in a cage for a special duty, only to find her body. Standing over her is a girl in an eyepatch who looks exactly like Asa and claims to be the true Asa, come to kill everyone.

Yuru is rescued by Dera, an easygoing merchant who frequently visits the village, and in a desperate moment, Yuru makes an offering to the village deity. This summons a pair of living stone komainu daemons named Left and Right, who have served as guardian deities for over four hundred years. They become bound to Yuru, granting him access to a world he never knew existed. The attack reveals that Yuru and Asa are the Twins of Destiny, separated as children and prophesied to control all daemons. Yuru is the Twin of Night with the unawakened power of Seal, while the real Asa is the Twin of Day with the active power of Break, which can forcibly sever all connections, including the contracts between humans and their daemons.

The story unfolds as Yuru descends from his destroyed rural home into the modern outside world, a place of cities, cars, and firearms that he does not understand. He is taken in by Dera and his partner Hana, a small but ferocious fighter who wields brass knuckles and acts as a grave digger for their organization. Together, they seek to reunite with Asa, who has been living under the protection of the Kagemori family. The Kagemoris are a powerful clan of daemon users, and their members include Jin, a clean-cut young man in glasses who serves as the family's assassin and cleaner, commanding anglerfish-like daemons named Ai and Makoto, and Gabby, a small hooded girl who acts older than she looks and commands a terrifying jaw daemon covered in eyes named Gabriel. While Asa has found a friend in Gabby and works as a manga assistant, she shares her brother's desperate wish to reclaim a normal family life, and she wears an eyepatch to contain the lingering effects of her power.

The narrative moves between Yuru learning to navigate a world of modern technology, corporate skyscrapers, and coffee shops, and the escalating conflict with the forces that want to capture the twins for their powers. The series balances moments of dark violence, as assassins and rival clans attempt to claim the twins, with the character-driven humor of a boy raised in a medieval village confronting escalators and mobile phones. Major arcs include the initial flight from Higashi Village, the introduction of the Kagemori clan's complex internal politics, and the gradual revelation of the twins' birth parents, who disappeared years ago under mysterious circumstances. As Yuru and Asa struggle to reunite, they must also uncover the truth about the legends, mysteries, and secrets of their village while fighting off enemies who see them not as people but as weapons capable of saving or destroying the world.