The Hardy boys are elated over their good luck when wealthy Elroy Jefferson invites them to spend Christmas vacation at his private retreat on Cabin Island. But when Frank and Joe make a reconnaissance trip in their iceboat the Sea Gull to the island, a belligerent stranger orders them off. Why? Before 24 hours have passed, the Hardys find themselves involved in two mysteries: the first concerns the recent disappearance of Mr. Jefferson's grandson, Johnny; the second, the baffling theft of a priceless collection of antique medals which took place two years ago. The young detectives, with their pals Chet Morton and Biff Hooper, pursue both cases on the icebound, snow-covered island. Sabotage to the Sea Gull, danger to themselves, and a ghostly prowler do not daunt Frank and Joe in their search for Johnny Jefferson and for clues to the stolen antique medals. How the teen-age investigators outwit a ruthless foe and succeed in solving both mysteries makes for mounting suspense in this brisk-paced adventure. - Flyleaf.