A research facility becomes a death trap, and only an untested Navy vessel can save the day in this adventure drama. A team of scientists led by Hal Hamilton (Michael J. Reynolds) is studying marine life in an underwater research station called Sealab. Shortly before the Sealab crew are scheduled to return to the surface, a massive underwater earthquake strands them at the bottom of the ocean. Project director Dr. Andrews (Walter Pidgeon), who had often fought to support the Sealab project against funding coordinator Norton Shepherd (Ed McGibbon), knows that he must act quickly to save the lives of those on board; he recruits Cmdr. Adrian Blake (Ben Gazzara) to use his new experimental submarine to find the Sealab and rescue the crew. Joining Blake on his mission are veteran sailor "Mack" McKay (Ernest Borgnine), his assistant Bob Cousins (Donnelly Rhodes), and Leah Jansen (Yvette Mimieux), a respected scientist and Hamilton's bride-to-be.