One summer, a 6-year-old girl named Hotaru Takekawa came to her grandfather's house for vacation. She broke into the forest of a mountain god that 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 Firefly to find the way home. Although Firefly is extremely grateful, the boy forbids her from touching his body. It turns out that the boy named Jin is not a human being, and he will disappear once touched by a human being. In the days that followed, Firefly and Jin became good friends and they traveled to every corner of the forest to play. Day after day, year after year, every summer, Firefly would come to the forest to meet her good friends as promised. She kept her agreement with Jin and would not touch Jin's body under any circumstances. As they grow older, Ying and Jin's feelings for each other have quietly changed. They look forward to the days when they can get together and embrace each other...