Scilly Isles, shortly after WWI. Edward, a fisherman haunted by the war, rescues a mysterious girl floating on the morning water, singing in a language unknown to him and brings her ashore. Upon discovery, he soon learns that she isn't from Scilly, the mainland or from this world at all, but a mermaid named Annwn. An unbreakable bond forms between them and their fondness for one another deepens with each passing day. However, there is a growing sense of unease as reports of local people going missing from the Islands slowly begin to emerge...