A crumbling Scottish castle, a missing scientist, and suspicious lights in the night-Philip, Dinah, Jack, and Lucy-Ann thought their holiday would be quiet. But when they explore the castle ruins, they discover secret passages and odd carvings that suggest the place may still be in use. Kiki, as usual, is the first to sense trouble, but the others are quick to follow a trail that leads not to treasure-but to espionage. Who are the men meeting in the dungeons? And what are the strange noises echoing through the hills at night? The more they Uncover, the more dangerous it gets. Then, without warning, one of the children vanishes within the castle walls and the only clue left behind is a cryptic, half-burnt map.