The TARDIS materializes on an island where they are captured and taken underground.  They are taken to the city of Atlantis.

The Atlanteans use men and women that can breathe under water to farm food from the sea.  Professor Zaroff has told the Atlenteans that he will raise their city.  He is, instead, planing to destroy the Earth by draining the sea into the Earth's core.  The TARDIS crew, with some help, persuade the Fish People to stop farming the saes.  The Doctor is able to break down the sea walls and flood the city, killing Professor Zaroff.