Rainbow Fireflies

On a blazing hot day, a young boy named Yuta is visiting the Hotarugaoka Dam, place where his father passed away, when he encounters a strange old man. Yuta helps the old man by giving him something to drink. Later that day the mysterious old man repays his kindness by saving the boy and taking him to the past, one month before the area is unnocuppied for the construction of the dam. Unaware of why he is there, Yuta meets Saeko, a girl who loves fireflies.