[[caption-width-right:350:Behold: The failed attempt at replacing the Daleks.]]
-> ''The Doctor's almost as clever as I am.''
The first of many serials written by Creator/RobertHolmes.
The TARDIS lands on the planet of the Gonds, who are ruled by the crystalline Krotons, whose ship crashed there thousands of years ago. The Krotons are in suspended animation inside their ship, the Dynotrope, but use their mental powers to dominate the Gonds. The brightest Gond minds are periodically sent as tribute on board the Dynotrope where the Krotons feed from their mental energy and discard the husks.
The Doctor and Zoe take the Krotons' test, and their mental energy is enough to revive the aliens. The Doctor discovers enough about them to enable him to destroy them and their ship with sulphuric acid, helped by a Gond scientist.
* AttackAttackAttack: Eelek's answer to all problems, although fortunately he doesn't persuade many people.
* AuthorityEqualsAsskicking: When the dynatrope starts lifting off, the mature, distinguished hereditary leader breaks through two rows of guards and does a forward roll under the closing blast doors into the ship. While holding a big vial of acid!
* ButtMonkey: "Jamie's unusable!"
* ChekhovsBoomerang: The mica.
* DamselInDistress: Vara and later Zoe, when she too is chosen for a "companion".
* DistressedDude: Selris, the Doctor, and Jamie all fall into the Krotons' hands. To be sure, the later two escape on their own power, but it's close.
* DoomAsTestPrize: Becoming a companion of the Krotons involves being mindsucked and then melted with acid.
* [[DressedAllInRubber Dressed All In PVC]]: Zoe.
* HandWave: Explaining how the acid could dissolve the Dynatrope:
-->'''Zoe''': But the machine wasn't ''pure'' tellurium!
-->'''The Doctor''': Well, no, the acid wasn't pure sulfuric acid... But it worked!
* HeroicSacrifice: Selris
* HollywoodAcid: That sulfuric acid sure gives off a lot of smoke.
* HumanSacrifice: Invoked. The Doctor rejects this on the grounds they are too civilized.
* ImpendingDoomPOV: When the Kroton heads off to destroy the [=TARDIS=].
* KickTheDog: The fact that Vana recovers completely after being mind-sucked makes it clear that there was no reason beyond malice for the Krotons to murder their "companions". If they hadn't, not only would they have pissed off the Gonds and the Doctor much less, but they could have got their energy much faster by repeatedly harvesting from the same people.
* LargeHam: "I shall attack them with slings and fireballs!"
* MindRape: The Doctor and Zoe get this when they start using their minds to power their machines. This is represented in a fairly disturbing and abstract sequence of the two of them moaning in pain through a fisheye lens.
* NeverSayDie: The Krotons use the word "dispersed" for their victims and the word "exhausted" for their own kind. Everyone else does say "dead".
* OnlySmartPeopleMayPass: Used [[Recap/DoctorWhoS2E2TheDalekInvasionOfEarth again]] by the MonsterOfTheWeek to get people smart enough to serve them. In this case, brain power is used to propel a spaceship.
* TheQuisling: Subverted with Selris and Eelek. Selris is initially duped by the Krotons, but doesn't go into denial when their evil becomes apparent, and his reluctance to attack them is a realistic appreciation of his people's relative weakness and the fact that the Krotons would probably respond with genocide. Eelek, who in usual examples of this trope would be the heroic leader of a LastStand, is transparently unstable, power-hungry and villainous[[note]]For a start, he's played by Philip Madoc[[/note]].
* RoyalBlood: On this world, leadership is hereditary. Selris' objection to being deposed is due to this, and after his death, his son speaks of how he shall have to deal with the other man.
* SiliconBasedLife: The Krotons are tellurium-based.
* StandardFemaleGrabArea: Zoe, Vana and the Doctor are grabbed this way in episode 4. What makes it the standard ''female'' grab area, however, is the fact that the Doctor just shakes his captors off, while the girls don't.
* [[SiliconBasedLife Tellurium Based Life]]
* WardrobeMalfunction: At the start of episode 4, there's a fairly large rip in Zoe's jacket. In some later scenes, the Doctor [[NoSenseOfPersonalSpace keeps his hand on her shoulder]] -- in reality, to hold the damaged costume together.