Two boys, Yoshiki and Hikaru, share a close friendship in a small, isolated mountain village. One winter, Hikaru goes missing in the surrounding hills. A week later, he returns, seemingly unharmed. However, Yoshiki immediately senses something is wrong. The person standing before him has Hikaru’s face, voice, and memories, but the boy he knew is gone. The entity that now wears Hikaru’s body is something else entirely, an ancient, inhuman being that wandered the mountain for a long time without feeling anything until it encountered the dying boy and decided to inhabit his form. Confronted with this reality, Yoshiki makes a painful choice. Though terrified, he agrees to keep the entity’s secret, preferring this imitation of his friend to having no Hikaru at all. The being, experiencing human life for the first time, develops a deep attachment to Yoshiki, one that surpasses the original Hikaru’s feelings, but its understanding of life and death is fundamentally alien, casually stating it sees little difference between the two.

The story is set in the claustrophobic town of Kubitachi, a place with a dark history tied to a local god or entity named Nonuki-sama. Generations ago, a member of Hikaru’s family, the Indou clan, asked this god to revive a dead wife, leading to mass violence. As penance, the Indou family has since performed a ritual to confine the god to Mount Nisayama. The entity that now lives as Hikaru may be this god or something similar, and its descent from the mountain has broken a fragile barrier, causing the veil between the human world and the spirit world to thin. Impurities, the souls and emotions of the dead, begin seeping into the village, leading to increasingly violent and bizarre incidents.

Yoshiki is the protagonist, a normal teenager struggling with grief, fear, and a complex, likely romantic attachment to the being that has replaced his best friend. The entity called Hikaru is the other central figure, an eldritch being that is both innocent and dangerous, capable of great affection and casual murder. Asako Yamagishi is a schoolfriend with psychic abilities who becomes suspicious of Hikaru and nearly loses her life because of it. Rie Kurebayashi is a knowledgeable housewife and powerful psychic who helps Yoshiki understand the supernatural dangers and researches what Hikaru might be. Tanaka is a enigmatic agent who works for a mysterious company, sent to investigate the disturbances; he has his own agenda and is not afraid to perform extreme experiments on Hikaru, such as cutting off his head to observe his regeneration.

The narrative follows several key arcs. The Daily Life Arc establishes the unsettling new normal as Yoshiki and the Hikaru entity attempt to live alongside each other, with the entity protecting Yoshiki from other spirits while struggling to control its own homicidal impulses. The Mystery Solving Arc sees Yoshiki and Hikaru delving into the history of Kubitachi and the Indou family, attempting to uncover the entity’s true origins while being hunted by Tanaka. The ongoing Hole Closing Arc involves a concerted effort by Yoshiki, Hikaru, Rie, and others to seal the spiritual breach and stop the influx of impurities, forcing the entity to confront its own nature and the pain it causes by preventing the real Hikaru’s loved ones from mourning and moving on. Faced with this, the entity offers to return to the mountain, but a devastated Yoshiki finally admits he wants the being that is with him now to stay, regardless of what it is.