Because nothing says “galactic menace” like clunky robots that squeak when they walk.
Short Synopsis
The Doctor, Jamie, and Zoe land on a planet where an alien race called the Krotons have enslaved the locals with the old “pretend to be gods” routine. The Krotons demand the brightest students for “advanced learning” (translation: brain drain and death). Naturally, Zoe signs up, the Doctor bumbles in after her, and together they prove that even glorified kitchen appliances can be defeated with a little chemistry.
Extended Storyline
The TARDIS lands on a planet populated by the Gonds, a perpetually oppressed people who’ve been stuck in a rut for generations. Their rulers? The Krotons, alien machines that have been “guiding” Gond society for centuries—mainly by keeping them dim-witted, forbidding independent thought, and murdering anyone who dares to get too clever. (So, basically the education system, but with more vaporization.)
The Gonds believe the Krotons are benevolent, despite the fact that every year the “best and brightest” students are sent into the Krotons’ machine for “advanced education.” Of course, by “advanced education” the Krotons mean “hooking their brains up to a giant vacuum tube and then quietly killing them.” Surprisingly, no one notices the lack of alumni.
Cue Zoe, who, being Zoe, immediately aces the test and gets herself selected. The Doctor, deciding that letting his companion be liquefied isn’t great form, takes the test himself (and passes with flying colors, much to his delight). Jamie, of course, doesn’t bother with the test—he prefers the traditional Scottish curriculum of yelling at things and hitting them with swords.
Inside the Krotons’ crystalline lair, the Doctor and Zoe discover the truth: the Krotons are crystalline lifeforms who crashed on the planet and need intelligent minds to power their ship. Without brain juice, they just sit around like particularly ugly lawn ornaments. Once the Doctor figures this out, he uses his trademark wit (and a lot of chemical know-how) to whip up an acid strong enough to dissolve their crystalline structures. Goodbye Krotons, hello freedom.
The Gonds are finally free to build their society without alien overlords telling them not to read books, which sounds like a step up—though given their track record, they’ll probably invent multi-level marketing schemes within a week.