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 fromFoundation 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 fromit'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 BlakeUSN. 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.