Travelling through space, the TARDIS suffers a strange energy drain and crash-lands on the planet Exxilon. The Doctor and Sarah Jane Smith venture outside to investigate the cause of the interference, and become separated. The Doctor is captured by the planet's inhabitants - the savage Exxilons - but he escapes. Sarah is meanwhile attacked by one of the creatures in the TARDIS, and is forced to flee outside into the night. Later, she finds a huge and pristine white City with a flashing beacon.
When daylight arrives, the Doctor finds and triggers a booby trap - which causes a huge boulder to fall - and is found by a party of Marine Space Corps, who take him to their powerless ship. They are on an expedition to mine Parrinium - a mineral that is only abundant on Exxilon - which can cure and give immunity to a deadly space plague that is threatening the human colonies of the outer planets of the galaxy. The lives of at least 10 million men, women and children are dependent on the human expedition, who must obtain sufficient supplies of Parrinium and leave the planet within a month if they are save the people of those troubled worlds. The MSC ship too has been stranded on Exxilon due to the power drain. They show the Doctor some photos they have taken of the spectacular City nearby - which the Exxilons worship, sacrificing anyone who ventures too close to it. Sarah does so, and is captured and taken to the Exxilons' caves to be sacrificed by their High Priest.
Meanwhile, another spaceship arrives on Exxilon. The door opens, and four Daleks glide out, firing their weapons – which do not work. The Daleks' weapons have been affected by the power drain too, and their ship too is trapped. Because of the power drain, the Daleks, the Doctor and the group of humans form an uneasy alliance in their efforts to obtain supplies of Parrinium and escape Exxilon. Acting as the MSC group's spokesman, the Doctor learns from the Daleks that several of their own planetary colonies are suffering from the plague as well, and the Daleks need the Parrinium for the same reason as the humans. On their way to the humans' mining dome, the Exxilons ambush the party with bows and arrows. They kill one of the humans and a Dalek - which is defenceless without its power weapon - and take the others prisoners. The prisoners are taken to the Exxilon caves where the Doctor interrupts his assistant's sacrifice. Subsequently, he is given the death penalty too. When the duo sacrifice commences, a second party of Daleks - who have equipped themselves with mechanical, bullet-firing guns - attack in force, killing a number of Exxilons. They then force the Exxilons and humans to mine the Parrinium. The Doctor and Sarah flee into a network of underground tunnels - where the Doctor guesses that their sacrifice was intended to be completed by something within the tunnels. This is later revealed to be a mechanical snake - a 'root' probe that defends the City - which nearly kills the Doctor and totally destroys a Dalek that follows the Doctor and Sarah along the tunnels.
The Doctor and Sarah meet a group of underground-dwelling Exxilons - exiled from the others because of their different beliefs and objectives - lead by Bellal. Bellal explains that the City was built by the Exxilons' ancestors - who were once capable of space travel. The ancient Exxilons built the City to be capable of maintaining, repairing and protecting itself. However, fitting the structure with a brain proved to be a mistake; as the City no longer needed the Exxilons. On realising this, the Exxilons tried to destroy the City; but, instead, the City destroyed most of the Exxilons - leaving the tribe of savages and Bellal's group the only survivors. Bellal's group seek to complete the last act that their ancestors failed - to destroy the City, and ensure their race's survival - whereas the other Exxilons worship the City as a god. Bellal sketches some of the marking on the exterior of the City wall - which the Doctor recognises from an Earth temple in Peru. He deduces that the space-travelling Exxilons must have visited Earth, and taught the primitive humans how to build. Bellal also explains that the City supports itself through underground 'roots' and the aerial beacon. The Doctor realises that the beacon is the cause of the electrical drain, and decides to go to the City and resolve the problem.
The Daleks separately come to the same answer of the City's beacon being the source of the interference that has stranded the spaceships on the planet, and arrange for the creation of two mechanically-timed explosives to destroy it. A Dalek observes two of the humans climb the City to fix the explosives to the base of the beacon, but Galloway secretly keeps one of the bombs. Two other Daleks proceed to enter the City to make a scientific investigation of the superstructure. The Doctor and Bellal arrive first, the Doctor believing the City to be one of the 700 Wonders of the Universe, and manage to enter the City just before the Daleks appear. The two parties then slowly proceed through the City - the Daleks only just behind the Doctor and Bellal - passing a series of progressive intelligence tests. The Doctor deduces that the City has arranged this so that only lifeforms with knowledge equal or superior to the City's original creators would reach the brain at the centre of the superstructure, allowing the City to add the knowledge and science of the survivors to its databanks. On reaching the central chamber, the Doctor begins to sabotage the circuits of the City's controlling brain - to which the machine responds by creating a pair of Exxilon-like 'antibodies' to 'neutralise' the Doctor and Bellal. The pair are saved when the Daleks enter and fight the antibodies, and the Doctor and Bellal escape as the City's controls begin to malfunction - owing to the Doctor's interference.
When the bomb on the beacon explodes, all electrical power is restored. The Daleks order the humans to load the sacks of mined Parrinium onto their ship. Their true intention for collecting the Parrinium is to use the supply to blackmail the galactic powers to accept their demands - to which refusal would mean the deaths of millions of planetary colonists. On leaving Exxilon, the Daleks will fire a plague missile onto the planet; destroying all life and making future landings impossible, so the Daleks will have the only source of Parrinium. As their ship takes off, Sarah reveals that the Daleks only have bags of sand - she and Jill loaded the real Parrinium onto the Earth mission ship, which is now ready to takeoff. Onboard the Dalek ship, Galloway - who has smuggled himself and the second bomb aboard amongst the sacks of fake Parrinium - detonates the bomb, destroying the Dalek ship before it fires the plague missile. Back on Exxilon, the ancient and magnificent City finally disintegrates and collapses, the Doctor sadly commenting that the Universe is down to 699 Wonders.
Doctor Jon Pertwee (Third Doctor)
Companion Elisabeth Sladen (Sarah Jane Smith)
Duncan Lamont — Dan Galloway
John Abineri — Richard Railton
Joy Harrison — Jill Tarrant
Julian Fox — Peter Hamilton
Neil Seiler — Commander Stewart
Arnold Yarrow — Bellal
Roy Heymann — Gotal
Mostyn Evans — High Priest
Michael Wisher — Dalek Voices
John Scott Martin, Murphy Grumbar, Cy Town — Daleks
Writer Terry Nation
Director Michael E. Briant
Script editor Terrance Dicks
Robert Holmes (uncredited)
Producer Barry Letts
Executive producer(s) none
Production code XXX
Series Season 11
Length 4 episodes, 25 minutes each
Originally broadcast February 23–March 16, 1974