Akuru no Shima

The setting is Aonokushima, a remote island in the Pacific Ocean far from Honshu. There, Reigo Murasame, a high school student, lives alone. His parents died in an accident, and his grandmother, who took him in to the island, also recently passed away. Still, he has no intention of leaving the island. Because he has someone he has feelings for, Ichika Kannagi. One summer day, the two of them decide to go to a summer festival. When they learn that there is a secret festival that is held exclusively for adults, they head off to a place deep in the mountain that is off-limits. Unbeknownst to me, that was the beginning of a tragedy.

(Source: Kodansha, translated)