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.
But the crew, having survived their ordeal, don their scuba gear and leave the bell. Instead of reaching the surface, they surface in a cavern. The crew members explore the cave and find a skeleton, then a disheveled old sailor named Matheny (George Skaff) who tells them that he and another sailor suffered a shipwreck fourteen years prior and has been living in these caves ever since. He claims there is no way out and a volcano provides air from the surface.
Meanwhile, up on the surface Prof. Wyman's younger brother builds another bell and launches it in an attempted rescue mission. However, with the old man in the cave starting to leer lecherously at the women, and the volcano growing more unstable, the second mission may not find them in time. The old man reveals to the women that he murdered the other sailor years ago, increasing their apprehension. Just as he is about to assault one of the girls, the volcano erupts and the old man is crushed under falling rocks. The stranded crew of the diving bell make their way up toward the surface and are rescued.
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