To You in the Beyond

Kazuki Tsukigase transfers to a high school on a remote island to get away from anyone who knows him. Kazuki discovers an inlet on the island where people are rumored to vanish. Exploring the inlet, he encounters an unconscious girl. When she comes to after Kazuki takes her to the clinic, she can't say anything about her identity except that she is named Nao, 16 years old, and whispers the year "1974." <em class=de-emphasized>(from manga)</em>

Synopsis from Anime News Network