Nijiiro Hotaru: Eien no Natsuyasumi

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.