A vacation along Panama's Pearl Islands led archaeologist James Delgado to an astounding discovery, the world's first working submarine, rusting on a remote beach, forgotten for over 135 years. We follow Delgado as he uncovers the mystery surrounding this maritime treasure. How did the vessel work, what killed its inventor and crew, and why did it end up here in this deserted archipelago? We also trace the amazing life of German engineer Julius Kroehl and how he invented a diving craft that was half a century ahead of its time.