One summer, a 6-year-old girl named Takekawa Hotaru came to her grandfather's house for vacation. She broke into the mountain god forest, which was said to be full of monsters. Just when she was anxious because she was lost, a big boy wearing a fox mask appeared in front of her and led Hotaru to find the way home. Although Hotaru was very grateful, the boy forbade her to touch his body. It turned out that the boy named Yin was not human. Once he was touched by humans, he would disappear. In the days that followed, Hotaru and Yin became good friends. They walked through every corner of the forest to play. Day after day, year after year, every summer, Hotaru would come to the forest as promised to meet her good friend. She abided by the agreement with Yin and would not touch Yin's body anyway. As they grew older, Hotaru and Yin's feelings for each other changed quietly. They looked forward to the day of reunion and hugging each other together... This film is adapted from the original work of Midorikawa Yuki.