Nijiiro Hotaru: Eien no Natsuyasumi
English title: Rainbow Fireflies
Japanese title: 虹色ほたる~永遠の夏休み~
Alternative titles: Niji-iro Hotaru: Eien no Natsuyasumi, Rainbow-colored Firefly: Never Ending Summer Vacation, Niji-Iro Hotaru ~Eien no Natsu Yasumi~, Rainbow-Colored Fireflies: The Eternal Summer Vacation
Type: Movie
Source: Novel
Episodes: 1
Status: Finished Airing
Aired: May 19, 2012
Duration: 1 hr 44 min
Rating: G - All Ages
Studios: Toei Animation
Genres: Fantasy, Romance
Themes: Time Travel

Yuuta was 12-year-old boy, who had lost his father in the traffic accident one year ago. In the summer vacation, he visited a deserted dam deep in the mountains, where he had a good time with his father before. 

Suddenly a thunder storm occurred and he slipped on the ground. He lost consciousness and woke up to find a girl and an unfamiliar village. He time-traveled 30 years and reached a village, which sank at the bottom of the dam. 

This is Yuuta's precious memory of "another" summer vacation.