As the Seaview conducts a marine census, Captain Crane spots a mermaid. He brings her aboard and falls under her spell. A sea creature follows the mermaid and proceeds to cause big trouble on the sub. The problems are compounded when the Seaview is ordered to disarm a nuclear device that has been planted off the coast of California.
Elenco: Richard Basehart , David Hedison , Robert Duvall , Terry Becker , Del Monroe , Paul Trinka , Richard Bull , Robert Dowdell .