 On vacation in the Scottish Highlands to visit his Uncle Michael (with Sally and Mildred in tow; Enright comes over later), Mac walks onto the grounds of the McMillan castle and shouts for Uncle Michael. The response is a rifle shot. Mac rushes in and finds Uncle Michael s lawyer and his housekeeper, who are trying to break into the "keep" which Uncle Michael uses as an office. Uncle Michael is dead on the floor, shot through the head with a rifle lying beside him. All the evidence points to suicide, but Mac won t believe it -- especially when two men run away from the castle on consecutive nights. Mac finds that the McMillan family has a centuries-long feud with the neighboring MacCready family, dating back to when a McMillan soldier caught a MacCready hiding in a secret passage on the eve of a battle -- and walled him in alive. The lawyer and several others in the case are MacCreadys, and perhaps one of them decided to finish the feud. Uncle Michael s irresponsible grandson soon arrives, inheriting the castle and, it s rumored, a hidden treasure. The grandson is also running the Highland Games, which spark monster sightings in a nearby "loch" (lake) similar to "Nessie." While hunting, Mac ducks several rifle bullets, and Sally dodges a huge man who practices caber-tossing -- at her. It s clear that someone believes in the existence of the treasure and thus has a perfect motive to kill Uncle Michael -- but who? The mystery deepens when Enright and Sally find that a wall of the keep has been recently re-mortared and the corpse of the long-dead MacCready is inside his secret passage. Is it a clue to the locked-room mystery, or another red herring?