It was just supposed to be a day at the beach, with some swimming and snorkeling, in the clear blue waters surrounding a small Bahamian island. But then everything changed. Sally was the first to see it - a dark shadow on the ocean floor. Carefully, Sally, Jimmy and their new friend Sandy, swam towards the shape in order to get a better look and were startled by what they saw. What the heck is an old, World War II submarine doing in the waters of the Bahamas? That's just one of the questions that 13-year-old twins, Jimmy and Sally Brant need to answer as they find themselves in the middle of a mystery. Follow along as they discover clues, dodge danger, and even rescue a dolphin as they use all their skills - and some help from the U.S. Coast Guard - to solve the mystery of The Sinister Submarine.