The one with menacing tile flooring.
Landing on the planet Exxilon, the TARDIS is drained of energy by a mysterious force. Emerging to investigate, the Doctor meets an expedition from Earth who are in search of Parrinium, a rare substance only found on Exxilon and which is vital for the cure of a virulent space plague.
Sarah, meanwhile, wanders off to investigate the tall building with a flashing beacon on top that she's spotted. She is promptly captured by a group of primitive Exxilon natives and taken off to their cave to be sacrificed for defiling the city.
The Doctor and the humans meet a group of Daleks, who have had their weapon energy drained by whatever mysterious force has disabled the TARDIS. They enter an uneasy alliance and then are also captured by the Exxilons.
The Doctor saves Sarah from execution, only to be sentenced to death himself. They both escape into the tunnels as more Daleks show up with mechanically-powered guns and attack the cave. The Doctor and Sarah meet Bellal, an intelligent, articulate Exxilon, who tells them that the Exxilons built the city themselves but brought about their downfall. The Doctor figures out that the tower Sarah saw is causing the energy drain, and decides to put it out of action.
The Doctor and Bellal make their way past a series of deadly traps through the city and the Doctor tinkers with the master computer to give it something akin to a nervous breakdown.
The Daleks have been busy stocking all the parrinium into their ship, and now take off, only for the ship to blow up in mid-air, thanks to a bomb smuggled on board by one of the humans, and Sarah reveals that she and another of the humans have replaced the Daleks' parrinium with bags full of sand, and smuggled the real thing onto their own ship. The city melts, and the Doctor is left to mourn the loss of one of the wonders of the universe.
An official BBC radio play by Barry Letts, "The Ghosts Of N-Space", takes place between this serial and the next and stars Three, Sarah Jane and the Brigadier.
- A.I. Is a Crapshoot: The ancient city.
- Ancient Astronauts: The Exxilons' forefathers built the temples in Peru.
- Big Bad Ensemble: Although the Daleks are the most visible antagonist, the City is actually the biggest obstacle to the Doctor and the humans, to the point that they briefly end up in an Enemy Mine with the Daleks. The Exxilons, while worshipping the City, act as an independent threat until the Daleks enslave them.
- Bioluminescence Is Cool: Bellal and his companion have glowing white vertical streaks on their torsos and thighs, although it's not entirely clear if it's this trope or something they're wearing to illuminate their subterranean surroundings (e.g. luminous body paint, smocks with lights built in).
- Black Comedy: The Daleks test their projectile weapons against miniature TARDISes.
- Cutting the Knot: The Daleks, who obviously can't solve the floor puzzle, just shoot it.
- Defector from Decadence: Bellal and the other underground Exxilons.
- Dies Wide Open: Commander Stewart.
- Driven to Suicide: The Dalek who let Jill escape just can't live with it...
- Dying Moment of Awesome: Galloway, after acting like a treacherous coward, stows away into the escaping Dalek ship and blows it up.
- Enemy Mine: Until the Daleks get functional weapons, at which point they break the alliance. No one was very surprised.
- Evil vs. Evil: The Daleks versus the City.
- Fanservice: Why else does Sarah start the story in a bikini?
- Genius Loci: An evil one.
- Ghost City: The Exxilon city.
- Heroic Sacrifice: Galloway sneaks into the Dalek ship and blows it up from the inside.
- Human Sacrifice: The Exxilons sacrifice anything that comes close to their temple, including Sarah Jane and the Doctor, of course.
- MacGuffin: in the form of a Magic Antidote
- Only Smart People May Pass
- Redemption Equals Death
- Rock Beats Laser: The humans' and Daleks' energy weapons, being drained of power, are no match for the Exxilons' spears and bows. Subverted when the Daleks re-equip with sub-machine guns.
- Sci-Fi Writers Have No Sense of Scale: The Doctor notes that if the Daleks succeed, they could kill ten million people across the galaxy. Ten million people is roughly the population of London, New York or Seoul. Pretty bad if you happen to be one of the ten million, but a drop in the ocean compared to the population of even some countries, let alone a galaxy.
- The Smurfette Principle: Jill, but she is actually the only civilian on the mission.
- Space Marine: Though in name only — if they had any Powered Armor or Cool Guns they wouldn't work.
- Spoiler Title: The Daleks' appearance is the first episode cliffhanger... except they're in the title already. Also, the title accurately predicts that Daleks will die in droves over the course of the serial.
- There Is No Kill Like Overkill: The Daleks seem to believe in this, considering that the first time they try out their machine gun weapons on some attacking Exxilons, a single round of shots not only kills them but sends their bodies flying. From this, the Daleks conclude their replacement weapons to be "moderately efficient".
- Villainous Rescue: Just as the Doctor and Sarah Jane are about to be sacrificed, the Daleks turn up and start shooting the Exxilons.
- Well-Intentioned Extremist: Galloway will do anything to make sure the mission is completed. Whether it involves killing the Exxilons, letting the Doctor and Sarah Jane die, or pulling a Heroic Sacrifice.
- What Cliffhanger: Episode 3 ends with the Doctor and Bellal about to walk into a room when the Doctor tells him to stop, pointing at the white and red tiled floor for no immediately apparent reason. It was the result of poor pacing, as the episode was scripted to end on a scene of the Daleks getting closer a few minutes earlier.
- What Measure Is a Non-Human?: Galloway doesn't consider "primitive" life worth saving. Also typical Dalek behaviour.
- The Worf Effect: Applied rather humiliatingly to the Daleks. At first, it's a plot point, where their technology has been disabled and they can't fire their signature guns, but thereafter it starts getting ridiculous. Apparently, their shields and defence mechanisms are kaput, too. Whacking them with swords causes them to explode, metallic hoses strike them just once and their heads deflate in a burst of flames, they get beat up by living computer programs, and one Dalek even kills itself simply because it failed to guard its prisoners (to be fair, Daleks do not tolerate failure and WILL exterminate their own if such happens, so this one was likely preemptively offing itself in the wake of the inevitable punishment). Even their Leitmotif for this story seems to be mocking in tone, with laughably Narmy trumpet horns.