The one with Buddhism.
The Doctor is off on another mission for the Time Lords (who presumably figure that giving him busywork will keep him out of trouble) - he has a sealed message to deliver to recipients unknown aboard a space station orbiting the planet Solos in the 30th century. Solos is about to gain independence from Earth's Empire, but the Marshal in command is determined to prevent this and is working with scientist Jaeger to convert Solos's atmosphere into an Earth-like one which will kill the Solonians.
The Earth administrator is murdered on the Marshal's orders and Ky, a young Solonian leader, is framed and flees to the planet, taking Jo with him. The Doctor follows, and his message pod opens for Ky, evidently the intended recipient. The contents of the pod turn out to be some ancient carved stone tablets.
Exploring the Thaesium mine in which they find themselves, the Doctor meets a human scientist called Sondergaard, who is searching for a cure for the mutating disease affecting the Solonians. The tablets turn out to be information about the history of the Solonians - apparently the mutations are part of a natural cycle and not a disease at all, and the radiation from the mined Thaesium is part of the cycle.
The Doctor takes a crystal from the mine and returns to the space station to use the laboratory, but is captured by the Marshal along with Jo and Ky and forced to complete the atmosphere converter. Sondergaard gives the crystal to Ky, who mutates first into a hideous creature, then an ethereal angelic being (apparently the ultimate mutation of the Solonians). The Doctor once again sabotages the machine, killing Jaeger, and Ky evaporates the Marshal.
- Angelic Aliens: Ky's final form is a floaty glowy person dressed in flowing robes.
- Antimatter: The Doctor claims that an antimatter explosion will turn everyone into "un-people un-doing un-things un-together," which is an awfully convoluted way of saying "dead."
- Applied Phlebotinum: Thaesium is radioactive rocket fuel and the main reason why humans want to colonize Solos, but it's also essential in the Solonians' mutation cycle.
- Arcadia: Ky assures us Arcadia was the Back Story of his planet.
- Author Tract: This story was a big attack on Apartheid and colonialism.
- Bald of Awesome: Sondergaard, especially if, as seems likely, his baldness is due to radiation exposure.
- BBC Quarry: Of course. Beady-eyed viewers will recognise the primary outdoor location, which is now occupied by Bluewater.
- Beauty Equals Goodness: Initially critiqued, but finally embraced with Ky's completion of the mutation cycle.
- Big Bad: The Marshal.
- Bizarre Alien Biology: The truth behind the Solos natives' mutations: they're undergoing a natural metamorphosis that'll allow them to survive their world's cyclical climactic shifts, which was triggered early due to the Marshal's terraforming efforts.
- Book-Ends: The story begins with the Doctor and Jo Grant stepping out of the TARDIS with a warning announcement and ends with them stepping back in with that same announcement.
- The Caligula: The Marshal.
- Conspicuously Public Assassination: The colonial Marshal has one carried out on the Administrator from Earth sent to give the subjects their independence.
- Continuity Nod: Jo uses her escaping skills again.
- Continuous Decompression: With that large a hole in the spacebase, the room should have been drained of air in seconds at most.
- Does This Remind You of Anything?: Heavily inspired by the UK's withdrawal from its former African colonies, and in particular the mess in Rhodesia, where the colony's white ruling classes declared independence early without the UK's permission (but with the support of some of the Conservative Party's hard-right wing) and attempted to impose a South African-style apartheid regime. (In real life, this did not end well.)
- Dying as Yourself - Varan's and his followers' plan, when they think that mutation is a bad thing.
- Everyone Calls Him "Barkeep" - they really go for it in this story, with the Marshall, the Administrator and the Investigator. It's not just the humans - Varan's son doesn't get a name either.
- Evil Colonialist: As a commentary on the UK's former colonies, this is to be expected.
- Evolution Power-Up: The Solonians mutate into new forms within their lifetimes, something which happens whenever their planet enters a new half-millennium-long "season". The Doctor at least notes this to be a unique lifecycle.
- Evolutionary Levels - with a twist on the usual human-centric scale. Here Human Aliens < Insectoid Aliens < Energy Beings
- Explosive Decompression - Averted: Varan doesn't explode when he's blown out into space. (Cue viewers who are used to this trope deeming it inaccurate.)
- Fantastic Racism
- Fantastic Slurs: "mutt" (originally intended to be "munt", a real ethnic slur used by English-speaking people in southern Africa against black Africans. It was apparently changed not because that was a bit too on the nose, but because it sounded like something else.)
- Fat Bastard: the Marshal
- Going Native - Sondergaard
- Green Rocks: "Particle reversal", used to explain just about every SF element in the plot.
- Hollywood Darkness: Even the darkest part of the cave was lit enough that the Doctor wouldn't have needed that torch.
- Hollywood Torches: Speaking of which, apparently a stick of wood from a camp fire burns with a foot-high flame for quite some time.
- Hostile Terraforming: The episode has a group of evil human colonists plotting to terraform an inhabited planet in a way that will genocide the indigenous sentient culture.
- Humans Are the Real Monsters
- I Love Nuclear Power - Zigzagged: The radiation jumpstarts the mutation process, which is actually a good thing. But radiation isn't enough, and because the process is started too soon, the Solonians don't have time to realise they also need the glowy crystal.
- Made a Slave: Ky complains this has been done to his people.
- Motive Rant - a short but effective one, when the Doctor gets the Marshall to express himself about the mutants in front of the Investigator.
- Mutants - Naturally!
- But averted: It's not a mutation but a metamorphosis.
- Noble Savage - Ky, in particular.
- Obscured Special Effects: Ky's transformation from a Human Alien to an insect-like monster was done with a Stop Trick and fade with an extreme closeup of his hand only, saving full-body makeup.
- Occupiers out of Our Country
- One-Gender Race: No female Solonians are visible at any point. Then again, this is one of the few Doctor Who stories with no female guest cast members at all.
- Physical God - The final phase of the Solonians' metamorphosis.
- Poor Communication Kills: If the Earth administrator has just started his speech by saying he was granting Solos their independence, the whole plot would have been avoided (and he'd have saved his own life).
- Power Glows: The cave, the radiation chamber and the final mutation of the Solonians all glow.
- The Quisling - Varan, to start with.
- Recycled Title: The title The Mutants was used at one point in the development of the story generally known as The Daleks and is still used by some fans to refer to that story. (Ironically, both stories had the same director, Christopher Barry.)
- Reverse the Polarity - Particle Reversal can do anything.
- Rewarded as a Traitor Deserves: Varan's Son assassinates Ky on behalf on the Marshal. When he comes to the Marshal's office to collect the promised reward, the Marshal shoots him with a dart gun.
- Ripped from the Headlines: The story was written as a commentary on Apartheid.
- Shout-Out: Jaeger is named after actor Frederick Jaeger.
- Thaesium Pipe Passageway: Used to escape the radiation chamber.
- Thicker Than Water: Varan chooses his own son for the assassin, which he cites as proof of his absolute reliability.
- Third-Person Person: Varan. Seemingly a personal affectation, as the other Solonians don't do it.
- Title Drop: Of course 'Mutants' is dropped all the time, and at the end of the last episode, the Investigator asks...Investigator: Doctor... who did you say?
- Those Two Guys/Those Two Bad Guys: Stubbs and Cotton, the two security guards who chip in with observations throughout the story.
- What Measure Is A Nonhuman - lampshaded and attacked throughout.
- You're Insane!:The Doctor: Marshal, you are quite mad.
The Marshal: Only when I lose.