From the cover: Because George's father needs quiet as he works with another scientist, the Five are not wanted at Kirrin Cottage. But the young son of the visiting scientist owns an unused lighthouse at Demon's Rocks, a treacherous piece of coastline not far away. The Five and their new friend go to live at the lighthouse, only to find themselves in the middle of a mystery. Who has broken into the lighthouse and why? Where does the deep shaft below the lighthouse go? And what does the lighthouse have to do with rumors of lost treasure, stolen from wrecked ships?