Into the Forest of Fireflies' Light

As a child, Hotaru lost herself in a forest near her uncle's place that she visits every summer. There, she had a fateful encounter with a spirit, who helped her even though he would disappear if a human being touches him. She began to visit him every summer, even as she grows; while he waits for the summer to come, for their time together to flow again.

Synopsis from Anime News Network