The TARDIS runs afoul of an energy drain and lands roughly on the planet Exxilon. The interior of the TARDIS in darkness is quite creepy. The planet outside is no better--desolate and scary and filled with rocks that seem to be shaped like living creatures. The Doctor and Sarah become separated. The Doctor is attacked by savage Exxilons, and Sarah stumbles on a great white city topped with a flashing beacon.
The Doctor is found by a party of Space Marines and taken to their ship, which has been drained of power like the TARDIS. Their expedition is to find a supply of parrinium, abundant only on Exxilon, which can cure a deadly space plague. Ten million people will die unless they can return with the mineral. They show the Doctor pictures of the great city, which the Exxilons worship, sacrificing anyone who enters. Sarah, of course, is taken to the Exxilons’ High Priest to be sacrificed.
A strange ship arrives, drained of power, and Daleks roll out. They are unable to exterminate the Doctor because the same power has drained their weapons. Their puzzlement, when they are unable to kill everything in sight, is amusing. It seems they too are looking for parrinium because their planets are suffering from the same plague. The Daleks, the Doctor, and the humans strike an uneasy alliance to obtain the rare mineral and escape the planet.
The Exxilons—clad in rags that make them look just like the rocks around them—may be primitive, but they manage to kill humans and Daleks and take all the rest into the cave. The Doctor interrupts Sarah Jane’s sacrifice and is condemned to the same fate. The Daleks have managed to replace their malfunctioning energy weapons with what are basically machine-guns. They attack, kill Exxilons, force them and the humans to mine parrinium. The Daleks plan to use this to generate a worse plague.
There are dissident Exxilons, who do not worship the city or indulge in sacrifice. They hide out in caves, hiding from the other Exxilons and from deadly long-necked creatures rather like the three-eyed tentacle in the U.S. movie version of War of the Worlds. Their leader Bellal, (Arnold Yarrow), tells Sarah and the Doctor that Exxilon was once a powerful advanced society, who built a self-repairing city with a brain. This led to their downfall. The Doctor realizes that the Exxilons once visited primitive Earth, that the beacon is what has crashed all the spacecraft, an that none of them can leave until the beacon comes down.
Bellal and the Doctor, working very nicely together, penetrate the city, solving various puzzle traps as they go, followed by two Daleks, who are a little less intellectual in their approach. The tests are to make sure only advanced intelligence can get to the city’s brain. The Doctor and Bellal gain access to the central chamber and sabotage the brain. The machine creates “antibodies” to attack them, but the Daleks destroy them.
The Daleks use two humans to plant bombs to destroy the beacon and order the slaves to load all the parrinium onto their ship. They plan to destroy everyone and everything on their way out. But the humans have loaded only sand on the Dalek ship and put the parrinium on their own. One of the humans has stowed aboard the Dalek ship with one of the bombs. The city disintegrates and collapses, the Daleks are destroyed, and the humans and time-travellers are free to go.
The story received mixed reviews, but I rather liked it. The music was a little too humorous for a Dalek story, though their consternation at finding themselves unarmed was rather amusing—a later incident in which a Dalek, under stress, panicked and self-destructed was strange to say the least. The planet was atmospheric throughout. The relationship between the Doctor and the diminutive, shrouded, blank-eyed Bellal was charming. Some suggested the Daleks didn’t need to be there at all, and indeed they were added later. On their ship, the Daleks test out their new weapons firing at tiny blue boxes. The symbol of the Space Force is the same as the symbol for Starfleet rotated ninety degrees.