Rainbow-colored fireflies, eternal summer vacation

One summer, the sixth-grade primary school student Yuta (voiced by Noble Takei) came alone to the Hotarugaoka Reservoir in the mountains. Back then, he and his father came here to catch unicorns. His father passed away unexpectedly a year ago. The reservoir where he had left countless beautiful memories naturally has a special meaning to him. During this period, he encountered a rare heavy rain and accidentally fell into the reservoir. Fortunately, Yuta was successfully rescued. In a quaint little village, he met the cute and innocent little girl Saeko (voiced by Ayumi Kimura). Occasionally, Yuta discovered that he was living in the 25th year of Showa (1950), and this was the small village that later sunk into the reservoir. What does it mean for Yu Tai to accidentally travel through time and space for thirty years? The film is adapted from the original work of the same name by Internet writer Kawaguchi Masaki.