The Doctor and his friends find themselves in the lost city of Atlantis, where the crazed Professor Zaroff has convinced the people of Atlantis that he can raise their sunken city from beneath the sea. However, the Doctor discovers a terrible secret behind Zaroff's plan — a secret that could destroy all life on Earth.

The TARDIS arrives on an extinct volcanic island. Before long, the travellers are captured and taken into the depths of the Earth, where they find a hidden civilisation — the lost city of Atlantis.