As butterflies ominously proliferate into town, the rumour of a mysterious creature lurking in the tunnel behind the school spreads among the children. When the body of Ari Kimura’s mother is found by this tunnel’s entrance, next to apparently human traces, the legend seems to be confirmed. Is the end of the world coming?

In order to appease the wrath of the beast, the children decide to offer it a sacrifice: The unfortunate Ari, whom they believe to be the cause of the curse, is shoved into a well that leads to the Nijigahara tunnel.