Niji-iro Hotaru: Eien no Natsuyasumi

Niji iro Hotaru Eien no Natsuyasumi Summary:

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.