Professor Millard Wyman (John Carradine) sends a crew of two men, Paul Whitmore (Allen Windsor) and Craig Randall (Robert Clarke), and two women Lauri Talbott (Sheila Noonan) and Dale Marshall (Phyllis Coates), down to ocean depths never before explored. But, there's a technical problem during the launch. The diving bell breaks free of the cable connecting them to the surface and loses contact with the surface. The mission is believed lost.