Human female has escaped, I have failed. Female prisoner has escaped, I have failed, I have failed. Self Destruct, I have failed! (starts spinning around uncontrollably and breaks down as it self-terminates) Destruct-I have failed-destruct-FAIL-DE-FAIL-DE-FAILED-Iiii-IIiii-IiiiI-AARrR-ArrRR-RRRrr-RRrrr-AGGGGHHH... (comes to a complete stop, utterly dead)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.
— A Dalek, dying of Plot-Induced Stupidity
An official BBC radio play by Barry Letts, "The Ghosts Of N-Space", takes place between this episode and the next and stars Three, Sarah Jane and the Brigadier.
- Adaptation Expansion: In the novelisation, the heroes attempt to reach their ships before the Daleks launch the missiles. Bellal has dialogue commenting on the destruction of the City (on screen he is silent during the closing sequence) and there is extra dialogue between the Doctor and Sarah at the end where the Doctor wants to resume the journey to Florana but Sarah just wants to go home.
- A.I. Is a Crapshoot: The ancient city.
- All There in the Manual: In the novelisation, Galloway is said to have lost his entire family in the Dalek wars, grown up in a refugee camp and worked his way up through the ranks of the Space Corps. He feels Stewart has been blocking his promotion because of disagreements on previous missions.
- It is specified that the MSC party was originally ten, with the others being lost in two ambushes.
- It is made clearer that the Daleks do not bother to supervise Galloway onboard their ship.
- 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.
- Big Damn Villains: Just as the Doctor and Sarah Jane are about to be sacrificed, the Daleks turn up and start shooting the Exxilons.
- 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...
- Enemy Mine: Until the Daleks get functional weapons, at which point they break the alliance. No one was very surprised.
- Evil Versus 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
- Ripped from the Headlines: The City of Exxilon is inspired by power cuts happening all around Britain at the time.
- 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.
- 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.
- The Squadette: Jill, but she is actually the only civilian on the mission.
- 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 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 defense 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 episode seems to be mocking in tone, with laughably Narmy trumpet horns.