Demon Bug

In the middle Heian period, a small island called Onishima is isolated from civilization.
As a young girl, Torago was playing with her sister, Tana, when she disappears into the sea unexpectedly.
However, it wasn't Torago's fault, but the fault of another boy who was playing with them.
Torago and that boy grew up with guilt, and later ended up together, but one day a woman who looks like “Tana” appears in Onishima...