The truth is not what you know. It's what you believe.

On the coast of Cork, Syracuse is a divorced fisherman who has stopped drinking. His precocious daughter Annie has failing kidneys. One day, he finds a nearly-drowned young woman in his net; she calls herself Ondine and wants no one to see her. He puts her up in an isolated cottage that was his mother's. Annie discovers Ondine's presence and believes she is a selkie, a seal that turns human while on land. Syracuse is afraid to hope again.