The motivating factor behind City Beneath the Sea is an underwater treasure hunt. In 1692, an underwater earthquake sends the Jamaica city of Port Royal to the bottom of the Caribbean. Three-hundred-fifty years later, deep-sea divers Brad Carlton (Robert Ryan) and Tony Bartlett (Anthony Quinn) take the plunge near Port Royal, in hopes of recovering a million dollars in gold bullion that was lost in a modern-day quake. What Brad and Tony don't know is that their employer, steamship magnate Dwight Trevor (Karel Stepanek), doesn't really want the divers to find the gold. Trevor hopes to pull off a major insurance fraud, and has no qualms about killing his divers to get what he wants. Mala Powers and Suzan Ball provide the love interest, while plump Hawaiian entertainer Hilo Hattie offers some welcome comedy relief.