Duchlan Castle is a gloomy place in the Scottish Highlands. Late one night the body of Mary Gregor, sister of the laird, is found in the castle. She has been stabbed to death in her locked bedroom. The only tiny clue to the culprit is a silver fish's scale, left on the floor next to Mary's body. Inspector Dundas is dispatched to investigate. The Gregor family and their servants are quick to explain that Mary was a kind and charitable woman but Dundas uncovers a more complex truth. Soon further deaths occur. Superstitious locals believe that fish creatures from the nearby waters are responsible; but luckily for Inspector Dundas, the gifted amateur sleuth Eustace Hailey is on the scene.