When a grizzled old man shoves a decades-old treasure map into 13-year-old John A. Macdonald's hands, he's certain this will be the best summer ever. But that was before a humpback lake serpent is seen at twilight by the people of Stone Mills in the mysterious Lake on the Mountain. As people flee in panic, John knows his family might be next unless he can figure out what dwells in the lake in time. But does he have what it takes to confront something he can't see clearly? Or will the serpent in the darkness win?