The Doctor and Sarah are on their way to the holiday paradise of Florana when the TARDIS's power abruptly fails and the ship materialises. Every electrical power source on the ship fails. The Doctor finds an oil lamp and he and Sarah investigate outside, opening the TARDIS doors with a crank handle. It is cold, dark and foggy and the Doctor investigates further as Sarah returns to the TARDIS to change out of her bathing costume. 

The Doctor is attacked by some natives and is dragged off. Sarah finds the lamp - it has blood on it - but there is no sign of the Doctor. The Doctor manages to escape his captors and, as daylight arrives, meets a small group of humans: weapons officer Lieutenant Dan Galloway (Duncan Lamont), Lieutenant Peter Hamilton (Julian Fox), Captain Richard Railton (John Abineri), and civilian geologist Jill Tarrant (Joy Harrison). Commander Stewart (Neil Seiler), the expedition's leader, has been attacked by the native Exxilons and is confined to bed. They are on the planet to get parrinium, a rare mineral which is needed to combat a disease that is sweeping the galaxy. 

Sarah sees a magnificent city and heads for it. Unfortunately, the city is sacred to the Exxilons and she is captured by them and taken to their High Priest (Mostyn Evans) who sentences her to be sacrificed. 

A spacecraft lands containing a squad of Daleks (Murphy Grumbar, John Scott Martin, Cy Town; voices: Michael Wisher). Their weaponry fails, however, and so they agree a pact with the humans until the power is restored. They all go to a mining area but are attacked, Railton is killed and the others are captured by Exxilons and taken to their temple. The Doctor rescues Sarah but they are all placed in a cell while a Dalek negotiates with the High Priest. 

Back at the Dalek ship, the Daleks test a new projectile weapon system. It is satisfactory so the Daleks stage an attack on the Exxilon temple. 

Commander Stewart, who has been brought from the camp by the Exxilons, tells Galloway that Hamilton is appointed as commander over him. He then dies and Galloway ignores his last request. 

In the middle of the sacrificial ceremony, the Daleks attack, allowing the Doctor and Sarah to escape into a tunnel system behind the altar. The Doctor explains that the Daleks move by psycho-kinetic power and so are not immobilised by the power drain. As they move on, they realise that something in the tunnels was intended to complete the sacrifice.

Galloway makes a deal with the Daleks that the Exxilons be used to mine the parrinium as long as the Daleks wipe out a breakaway group of Exxilons for them. 

Two Daleks follow the Doctor and Sarah down the tunnel. Sarah meets Bellal (Arnold Yarrow), a friendly Exxilon, and the Doctor encounters one of the living roots of the city, which spits electrical energy. It attacks one of the Daleks which is destroyed. The Doctor returns to Sarah and Bellal who, along with another Exxilon, Gotal (Roy Heymann), helps them escape from the other Dalek. Bellal explains that their race built the living city which then decided it no longer needed its creators and banished them. Since then the race has degenerated into two factions; one who worship the city and one who wish to destroy it. Bellal draws the markings which appear on the walls of the city and the Doctor recognises them from a temple in Peru on Earth. 

The Daleks send two of their number to enter the city, while Galloway and Hamilton are made to scale the city walls and place two explosive charges at the base of a beacon atop the city in the hope that by destroying the beacon they will stop the power drain. 

Bellal takes the Doctor to the city. The Time Lord tells Sarah to ensure that the humans are ready to leave the moment that the power returns. Sarah leaves the Doctor and Bellal investigating the markings. The two Daleks arrive and chase the Doctor and Bellal, but the Doctor manages to open the door by tracing the symbol that is different from the others. The Daleks are left to solve the puzzle themselves. In the first anteroom, the Doctor traces a maze on the wall to open the second door. The next puzzle is a pattern on the floor. It is electrified, but the Doctor uses his sonic screwdriver to negotiate it. The Daleks are close behind but the 7000 volt charge does no serious damage to them. 

The next test is of the mind and Bellal threatens the Doctor. The Doctor again uses his sonic screwdriver to break the spell and they are allowed to move on. The final test is on their sanity but the Doctor and Bellal manage to survive. They enter the city's control room where the Doctor starts re-wiring the circuitry as the city starts to create two 'living' antibodies (Steve Ismay, Terry Walsh) to destroy the intruders. 

Sarah hatches a plan with Jill to replace the parrinium with ordinary rock. When a Dalek guard finds that Jill has escaped, it self-destructs. Galloway and Hamilton arrive at the beacon and place one of the charges - Galloway wants to keep the other one for use against the Daleks. 

The Doctor and Bellal are attacked by the antibodies but the Daleks arrive and the creatures turn their attention to them instead. The Doctor and Bellal escape as the city starts to die. 

The Daleks plan to leave with the parrinium as soon as the power returns and they get the sacks of what they believe to be the mineral loaded onto their ship by Galloway and Hamilton. In fact, the parrinium is on the humans' ship and the sacks are full of nothing but earth. 

The beacon is destroyed and the power returns. The Doctor, Bellal, Sarah, Jill and Hamilton watch as the Daleks prepare to leave. The Daleks intend to fire a plague missile at the planet once they are in space to ensure that it becomes uninhabitable. Galloway, however, has hidden himself on their ship, and once it is in space, he detonates the bomb, destroying the ship and killing himself in the process. 

Sarah notices that the city is dying and they all watch as it melts away, screaming as it is destroyed.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}