Rainbow Fireflies

Yuuta, a 12-year-old boy who lost his father in a traffic accident a year prior, visits a deserted dam in the mountains for summer vacation. It's the same spot where he spent happy times with his father before. While there, a sudden thunderstorm causes Yuuta to slip and fall, leading to a loss of consciousness. He awakens to find a girl and an unfamiliar village, only to discover he has traveled back 30 years in time, ending up in a village that lies submerged at the bottom of the dam.