Holly Mason, a gifted amateur artist, vanished from the seaside village of Tideswell a year ago. Now her older sister, Alice, returns to settle her affairs. It should be simple, but it's not. Paintings found in Holly’s studio reveal dark visions of witch trials and of a monstrous figure that rises from the sea.

As Alice digs deeper she uncovers disturbing truths. Violence erupts and she is almost drowned. She lives, but needs time to recover. Back home in the care of her husband she thinks she’s safe but here, too, things are not what they seem.