The one with evil cardboard boxes, and 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".
- Deus ex Machina: The Hostile Action Displacement System that teleports the TARDIS away when a Kroton tries to destroy it.
- 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.
- Electric Instant Gratification: The Teaching Machines reward correct answers by inducing feelings of pleasure in the user.
- 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 sulphuric acid... But it worked!
- Heroic Sacrifice: Serlis charges into the machine to bring the Doctor the acid he asked for. He gets it to him, but the Krotons kill him.
- Hollywood Acid: That sulphuric acid sure gives off a lot of smoke.
- Human Sacrifice: Invoked. The Doctor rejects this on the grounds they are too civilized.
- Idiot Ball: The Doctor tells Zoe that the learning machines are being used to brainwash the Gonds and tells her not to wander off. She immediately wanders off and plugs herself into the learning machine to try it out.
- Impending Doom P.O.V.: When the Kroton heads off to destroy the TARDIS.
- Karma Houdini: Eelek never receives any comeuppance for betraying the Doctor and Zoe to the Krotons.
- 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.
- Our Monsters Are Weird: The Krotons, which have a Tin-Can Robot appearance but are actually sapient tellurium crystals that are technically immortal, with the closest thing to death that they have being to 'exhaust' (turn into a gaseous state and leave their vessel). The implication (made explicit in the books) is that they can possess any machine into becoming their body. They are blind, power their machines with mathematical aptitude, Mind Rape people in order to get the power, breathe fluid through hose lines in their chest, and have inexplicable accents.
- People in Rubber Suits: The Krotons resemble walking cardboard boxes with a (sometimes spinning) cardboard diamond on top.
- 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 .
- Red Herring: Intended with the Krotons. The script refers to them as silver giants and much is made of the robot snake singling out the Doctor, meant to make the viewer think the Cybermen would be the villains. But it doesnt show up much in the finished product and the reason the Doctor is targeted is obvious (the Krotons camera shows him holding an axe).
- 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.
- Snark Ball: The Second Doctor, who is usually compassionate, easy-going and as modest as the Doctor ever gets, has one stuffed down his throat by Robert Holmes and spends most of the serial playing Insufferable Genius, subtly mocking the Krotons and Jamie and bickering with Zoe over whether he knows how to do everything or not. This has the effect of moving his characterisation to be quite close to the Fourth Doctor's, a character Holmes was pivotal in creating.
- 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.
- Starfish Aliens: The Krotons are living crystals who resemble robots and power their spaceships with mental energy. Even by Doctor Who standards, they're pretty weird.
- 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.
- Weakened by the Light: The Krotons have poor eyesight in daylight and so have trouble moving around.