For the construction of the world's first underwater city, Bob Gage (William Lundigan) is assigned to supervise the work for Tim Graham's (Roy Roberts) engineering firm. Once the necessary pilings have been sunk deep into the ocean's bed, Gage employs a team of crack frogmen to assemble the pre-fabricated parts and cells of the complex sub-sea honeycomb. And while completing the project for Dr. Junius Halstead (Carl Benton Reid), head of the Institute of Oceanography, Gage falls in love with the scientist's lovely niece, Monica (Julie Adams). When the underwater city finally is completed and ready for the inspection by a team from Washington, Gage learns of a powerful undersea current that is eroding the ocean floor beneath the site. But the disaster only reinforces Gage's belief in the importance of his work and, with Monica at his side, he pledges to rebuild THE UNDERWATER CITY.