Liveaboard skipper and amateur sleuth Cass Lynch is busy at marine college in Scalloway, until one night she finds an acquaintance dead in a doorway with her hand smeared with peat ash. Rumours spread of a strange ritual linked to the witches once burned in Shetland's ancient capital, and of a horned figure abroad in the night. At first Cass believes these to be mere superstition, until there's a second murder, and she begins to wonder if the devil really does walk in Scalloway ...