Marine scientists prepare to leave their underwater ocean lab after an extended stay performing oceanographic research.

Dr. Andrews (Walter Pidgeon) enlists experimental sub captain Adrien Blake (Ben Gazzara) to survey the damage and rescue the oceanauts.

He brings along chief diver "Mack" MacKay (Ernest Borgnine) and Dr. Leah Jansen (Yvette Mimieux), fiancée of one of the scientists.

Blake finds the lab has been ripped from its moorings and has tumbled down an unexplored, deep sea trench, presumably intact.

With the lab's reserve air supply dwindling, the team descends into the unexplored trench and finds an incredible ecosystem populated with monstrously oversized fish.