Marc and his twin sister, Terri, are excited about spending the summer at their uncle Nicholas' lake house. Even though their uncle is an old grouch, Marc and Terri,enjoy the lake and their summer friends in Grove Hill. But this summer is different. Suddenly the once placid lake has motorboats, fishermen, and crowds. Even worse is the gross green slime that's been popping up on boats and docks and swimmers. When Terri goes for a swim and doesn't return, Marc wonders what exactly is lurking under the water....