The travellers find themselves deep below the Earth's surface, in the lost city of Atlantis. The Doctor meets the mad Professor Zaroff and Polly is to be turned into a fish person.
Jamie and Ben are set to work in the mines but are determined to escape. The Doctor discovers that Zaroff's mad plans will cause the destruction of the Earth.
As Zaroff's insane scheme to destroy the world nears completion, the Doctor plans to break down the sea wall and flood Atlantis.
The Doctor hatches a dangerous plan to kidnap Professor Zaroff and encourages the Fish People to revolt against their masters and put a stop to the production of food.