Aboard the R/V Triton, the Project Neptune team is doing oceanographic research. Director Andrews is trying to keep the research going in spite of opposition from Foundation Head Sheppard. Below on the ocean floor, in the Sealab, the team led by Hamilton is about to return to the surface when the Sealab is ripped loose from it's moorings and sent careening into a trench. Trapped too deep for divers, the only chance is rescue by a new US Navy mini-sub, piloted by the arrogant Cdr Blake USN. Blake, Chief Diver MacKay, Diver Cousins & Dr. Jansen (Hamilton's fiance) dive in the mini sub to attempt the rescue of the trapped Hamilton & crew.