Six months ago, teenager Hikaru Indou went missing in the mountains near his rural village of Kubitachi. When he returned a week later, he seemed to be the same cheerful boy as always. But his best friend, Yoshiki Tsujinaka, knows the truth. The thing that came back is not Hikaru. It is an entity that found the boy dying on the mountain and simply slipped into his body, perfectly copying his memories and personality. When Yoshiki finally confronts the creature, it does not deny the accusation. Instead, it admits that this is its first time living as a human, that it likes Yoshiki a lot, and that it does not want to kill him. Faced with the loss of his real friend and unable to let go, Yoshiki agrees to keep the creature’s secret and continue their life as if nothing has changed.

The story is set in the isolated village of Kubitachi, a place surrounded by mountains where the barrier between the world of the living and the spirit world is unnaturally thin. The villagers have long told stories of Nonuki-sama, a god said to dwell on Mount Nisayama, and the Indou family has secretly performed rituals for generations to contain the supernatural forces there. Now, with Hikaru’s body walking among them, those forces begin to leak into the village. Strange deaths occur, ghostly apparitions known as impurities appear with increasing frequency, and the fragile peace of the town begins to crumble.

The entity wearing Hikaru’s face is a being of immense power but no human morality. It does not truly understand the difference between life and death, and when an elderly woman recognizes it as Nonuki-sama, it kills her without hesitation. Yet it genuinely cares for Yoshiki, the first person to show it kindness. For his part, Yoshiki is trapped between his grief for the friend he lost and his growing feelings for the creature that has taken his place. He is terrified of it but also unable to abandon the only version of Hikaru he has left.

They are not alone in their secret. Asako Yamagishi, a sharp-eyed classmate with psychic abilities, grows suspicious of Hikaru and nearly pays for it with her life. Rie Kurebayashi, a powerful housewife who can sense and banish spirits, recognizes the entity for what it is and becomes an uneasy ally, explaining that Hikaru is not a ghost or a demon but something that exists beyond the cycle of life and death itself. They are also pursued by Tanaka, a mysterious investigator working for an unknown company who is fascinated by Hikaru and conducts disturbing experiments on him, including cutting off his head to watch him regenerate.

Over the course of the first season, the narrative unfolds in two major arcs. The Daily Life Arc follows Yoshiki and the Hikaru-entity as they attempt to maintain a normal existence, attending school and spending time together while supernatural incidents escalate around them. As Yoshiki begins to research the history of Kubitachi and the Nonuki-sama legend, the story moves into the Mystery Solving Arc. The boys uncover the truth about the Indou family’s failed ritual, the thinness of the spiritual barrier, and the fact that Hikaru was acting as a lightning rod for impurities when he lived on the mountain. By leaving, he has allowed those dangerous spirits to flood into the village.

The season concludes with the entity, having sacrificed half of itself to Yoshiki in an attempt to curb its violent instincts, realizing that its presence is preventing Hikaru’s family and friends from mourning and moving on. It offers to return to the mountain to draw the impurities away. But Yoshiki, finally able to articulate his feelings, breaks down and asks it to stay. He wants the creature itself, not the ghost of his old friend. The season ends with the two of them still together, the mystery of what Hikaru truly is unresolved, and Tanaka approaching them with new questions as the barrier continues to weaken.