The group in the intro are part of a multi-school extracurricular camping trip that studies history. Takuma Momozuka's group consists of his friend Minoru Hyuuga and Aoi Shibuya, who is one year older than the boys and from a different school.
They decide their project will be on "Kemonogami" (Beast God), which is a local-based-only religion, and visit the shrine that holds its rituals. The legend of the Beast God goes as far as the Kamakura period. A powerful clan that lived back in those days borrowed the power of a god, who lived in the mountains and was similar in appearance to a beast, to gain wealth, which earned its wrath so the clan was annihilated.
The locals were so afraid of the Beast God's anger that they gave their children to it as a ritual peace offering. Although this all happened long ago, whenever children enter the shrine that was used for this ritual even to this day, it's said that those children are spirited away.
On the morning of the 2nd day at their camp, the phenomenon approaches and Takuma, Minoru, and Aoi end up wandering into the world of this fading legend...