Duchlan Castle is a gloomy, forbidding place in the Scottish Highlands. Late one night the body of Mary Gregor, sister of the laird of Duchlan, is found in the castle. She has been stabbed to death in her bedroom - but the room is locked from within and the windows are barred. The only tiny clue to the culprit is a silver fish's scale, left on the floor next to Mary's body.