Summer at grandfather's cottage near Lake Ontario was always a thrilling time for Joanna -- but this summer promised even more excitement. In addition to her friend Mike with whom Joanna always explored the waterways and scattered islands, there was Hank, the very attractive newcomer who operated a floating vegetable store aboard a pickle boat. A peculiar flickering light from deserted Ecl Island arouses Joanna's curiosity and when several of their possessions begin disappearing including Mike's row-boat, the children embark on a sleuthing campaign. Local library research leads Joanna to the false conclusion that someone must be out for treasure abandoned by the original French settlers on Ecl Island. A few tingling explorations of the island itself are climaxed by the startling appearance of Hank and a frightened stranger conversing in heated German. The ""villain"", to everyone's relief and interest, is a boy stowaway from East Berlin, too frightened to turn himself over to the American authorities, but the kindness offered provides him with the necessary courage. The author's understanding of suspense techniques applied clue by clue blended with an authentic inspection of the Great Lakes region elevates this from the run of the mill mystery.