The Doctor's almost as clever as I am.The first of many serials written by Robert Holmes.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.
- Attack! Attack! Attack!: Eelek's answer to all problems, although fortunately he doesn't persuade many people.
- Authority Equals Asskicking: 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!
- Butt Monkey: "Jamie's unusable!"
- Chekhov's Boomerang: The mica.
- Damsel in Distress: Vara and later Zoe, when she too is chosen for a "companion".
- Distressed Dude: 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.
- Doom As Test Prize: Becoming a companion of the Krotons involves being mindsucked and then melted with acid.
- Dressed All In PVC: Zoe.
- Hand Wave: 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!
- Heroic Sacrifice: Selris
- Hollywood Acid: That sulfuric acid sure gives off a lot of smoke.
- Human Sacrifice: Invoked. The Doctor rejects this on the grounds they are too civilized.
- Impending Doom P.O.V.: When the Kroton heads off to destroy the TARDIS.
- Kick the Dog: 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.
- Large Ham: "I shall attack them with slings and fireballs!"
- Master Computer
- Mind Rape: 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.
- Never Say "Die": The Krotons use the word "dispersed" for their victims and the word "exhausted" for their own kind. Everyone else does say "dead".
- Only Smart People May Pass: Used again by the Monster of the Week to get people smart enough to serve them. In this case, brain power is used to propel a spaceship.
- The Quisling: 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 Last Stand, is transparently unstable, power-hungry and villainousnote .
- Royal Blood: 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.
- Silicon-Based Life: The Krotons are tellurium-based.
- Standard Female Grab Area: 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.
- Tellurium Based Life
- Wardrobe Malfunction: At the start of episode 4, there's a fairly large rip in Zoe's jacket. In some later scenes, the Doctor keeps his hand on her shoulder — in reality, to hold the damaged costume together.