Akuru No Shima summary:

The setting is Aonokujima, a remote island in the Pacific Ocean far from Honshu. There, a high school student, Reigo Muraame, lives alone. His parents died in an accident, and his grandmother, who took him in, recently died. Even so, he does not want to leave the island. He has a girl he is in love with, Ichika Kannagi. One summer day, they decide to go to the summer festival together. They are buoyed by the knowledge that there is a secret festival that is held exclusively for adults, and they go deep into the mountains where it is forbidden to enter. Little do they know that this is the beginning of a tragedy.