The Underwater Menace, Episode One

The Doctor and his friends materialise on a volcanic island, where they are kidnapped by natives of an underground civilisation. To the Doctor's great surprise, these are the survivors of Atlantis - and the famous Professor Zaroff, thought to have died twenty years ago, is amongst them. Zaroff has provided the Atlanteans with a new source of food, has taught them how to surgically convert their slave caste into Fish People who can survive underwater, and has promised to raise Atlantis from the waves. Only the Doctor realises that Zaroff is mad, and that his "plan" to raise Atlantis will in fact result in the destruction of the Earth.