Cass Lynch has been persuaded to spend Christmas in the Highlands with her friend DI Gavin Macrae, but their romantic walk by the loch is cut short when they find a skeleton among the bracken. Back home in Shetland, Cass hears about Ivor Hughson, who left his wife and failed business months ago, and hasn't been heard of since. A near-disaster aboard Cass's yacht suggests someone wants to stop her asking questions about his disappearance. Meanwhile, there are eerie reports of sightings of a njuggle, a Shetland water-horse which drowns curious passers-by. Soon it's taking Cass all her wits to stay alive ...