The Fish people

The TARDIS materialises on a rocky coastline which, when The Doctor, Ben Jackson, Polly Wright and new companion Jamie McCrimmon start to explore, is in fact an extinct volcanic island. It is not long before the travellers are captured and taken deep below sea level into the depths of the Earth, where they find a hidden civilisation - the lost city of Atlantis.

At first they receive a hospitable welcome and are escorted to a room containing a table laden with food, which The Doctor recognises is made from plankton. However, when a priest named Ramo enters accompanied by guards, The Doctor realises that they are to be sacrificed. When the others are taken away he requests that he is allowed to send a note to Professor Zaroff, whose involvement he has guessed is behind the plankton-based food. The Doctor’s request is refused but, before being led away, he persuades a young serving girl, called Ara, to take his note to Professor Zaroff.

Ben, Polly and Jamie are taken to a temple where they are bound and placed on three of four slabs surrounding a shark-infested well. Hoping that The Doctor will be able to rescue them they are dismayed when The Doctor is led in and placed on the fourth slab. However, the note he managed to get to Professor Zaroff prompts Zaroff to storm into the temple and demand the release of The Doctor – who in turn persuades Zaroff to also release his three companions.

Professor Zaroff and King Thous

But they are not safe yet. As The Doctor goes off with Zaroff, Ben and Jamie find themselves sent off to work in the mines while Polly learns that the plankton is harvested by humans who have been surgically altered to use gills to breathe under water. A fate which scientist Damon has planned for Polly so that she can become one of these fish people and so tend to the undersea food producing areas.

The Doctor soon discovers that Professor Zaroff has convinced the Atlanteans that he can raise their city from the sea, but actually he plans to drain the ocean, through a fissure in the Earth's crust, into the Earth's molten core. But The Doctor realises that by doing this the resultant superheated steam will cause the planet to explode. But rather than denying The Doctors warnings Professor Zaroff agrees with The Doctor’s premonition. This convinces The Doctor that Zaroff has become deranged when the scientist boasts that it will be his most magnificent achievement to-date.

The Doctor

The Doctor then learns of Polly's forthcoming operation and, after distracting Zaroff, is just in time to interrupt the electricity supply to the operating theatre, thereby postponing the conversion operation and so allowing Ara to rescue Polly.

Meanwhile Ben and Jamie are set to work in the mines with fellow prisoners Sean and Jacko where they learn of a plan to escape through a tunnel that they have discovered. This tunnel leads to a secret chamber that is behind a statue in the temple of Amdo. There they discover Ara and Polly who have been hiding there from the guards who are searching for them.

The Doctor, who has managed to escape from Zaroff’s laboratory, tries to warn Thous, The King of Atlantis, of Zaroff’s destructive plans. Instead Thous is convinced that Zaroff’s plan is to the good of Atlantis he orders that The Doctor and Ramo, who has helped The Doctor seek an audience with the King, to be taken to the Temple of Amdo to be executed. Luckily The Doctor is rescued by his companions and so they are all reunited at last.

Polly Restrained on Table

Knowing that the plankton-based food will not last long before perishing, The Doctor, with the help of Sean and Jacko, decides to cause a revolution by persuading the Fish People to stop farming so creating a food shortage.

The Doctor and his companions then head off to tackle Zaroff himself. The Doctor disguises himself as a gypsy soothsayer at the Atlantean market and helps trigger a ruse to separate Zaroff from his guards. This works and Zaroff is captured. But, after boasting that his plan to drain the ocean is unstoppable, he manages to escape and with his loyal guards he decides to enter the royal court and confront Thous. He finds though that the King is aggrieved by the strike among the Fish People and so has lost his faith in Zaroff to raise Atlantis from the sea. Zaroff responds by shooting Thous, while his guards take on the royal protectors in pitched combat.

The Doctor finds the injured King in the throne room and has him taken to the secret chamber for safety. After alerting the Atlantean populace to flee to the higher levels The Doctor, with help from Ben, cut through cables to render the Atlantean’s power reactor unstable. This causes the lower portions of Atlantis to become flooded including Zaroff’s laboratory. Obsessed with his scheme to destroy the Earth, and now totally mad, Zaroff is determined to carry out his plan but instead finds himself trapped behind a grill with the water rising - so preventing him reaching the detonation controls. The Doctor and Ben though are unable to help him escape from the rising water and so flee to the upper levels leaving Zaroff to drown.

Knowing that most of the Atlanteans, including the King, Sean and Jacko, will have survived, The Doctor and his three companions return to the TARDIS. Safe at last The Doctor operates the controls. However, just as the TARDIS dematerialises it is suddenly gripped by a violent force and goes out of control.