Much of Japan has been covered in ash following the eruption of Mount Fuji, and the particulate continues to fall from the sky and blanket the ground like snow. With a second eruption likely to occur, many people have fled, but there still remain those either unable or unwilling to leave, spending most of their time in their homes.

One of these left-behind people is Rui Satou, a high school student who tries to make friends online between classes. When he starts chatting with Michiru, the two quickly form a bond, and Rui decides to meet her in person despite the fact that she lives closer to the blast zone. However, after meeting Michiru, Rui discovers that their bond is stranger and more powerful than he could have imagined.