The one with Skeletor. Who then becomes Captain Camp.
Welcome to Traken, a world so perfect that evil can't survive! Seriously: any evil that tries to enter the atmosphere will literally crystalise out of the Trakenite air, ending up as a nugget of evil enclosed in a creepy humanoid statue with glowing red eyes.
And yet the Trakenites are good to the evil statues. They are so good that they assign people to look after the statues: bring them little bouquets of flowers, inquire after their families, persuade them to forsake wickedness, wipe off the bird poo, etc., until such time as the statue's residual sentience fades away, probably from sheer irritation. Then the stone crumbles into harmless dust and enriches the Trakenite soil.
But one statue was different. One statue contained an especially intractable malevolence, dubbed "Melkur", and its assigned guardian was a high-ranking Trakenite: Consul Kassia. Kassia's husband, Consul Tremas, was also a Traken elite, not to mention next in line for the Keepership; his daughter Nyssa was poised to carry on her family's august traditions. So, when Kassia was eventually corrupted by Melkur's malign influence, she was extremely well placed to wreak some serious damage to the Trakenite status quo.
Not that the planet's ruler, the kindly old Keeper of Traken, suspects The Mole in their midst. All he knows is that something is going pear-shaped, and he'd very much like the Doctor to drop by and help fix it. The Doctor is obliging, and soon he and Adric have made landfall—conveniently enough—right in front of Melkur!
They get hauled up in front of the Trakenite version of the Omniscient Council of Vagueness, where the Keeper is summoned again but only says "Evil! EEEEEEVIL!" Which, it must be admitted, does cast our heroes in a rather poor light. Thanks a lot, Keeper. Consul Tremas speaks up, protesting that the Doctor and Adric haven't been convicted of anything (yet, anyway); he gets rewarded by having them released into his custody and warned that his head is at stake if they misbehave.
Vicious group of people they got on this "perfect" world, isn't it?
Over in Tremas' quarters, we meet his daughter, Nyssa. She's been assigned to Melkur-visiting duty ever since her father and stepmother married, but she doesn't know that Kassia has been visiting it on the sly. Or that it's started talking to her, or given her a necklace that makes her evil.
Back in the Council Chambers, one of the interchangeable Consuls tries to commune with the Keeper to figure out what's going on, but the stress kills him.
Actually, Kassia kills him. Actually actually, the Melkur controlling her killed him, by shooting laser beams from her eyes.
Meanwhile, the Doctor, Tremas, Adric, and Nyssa run around trying to subvert the Melkur's evil plans, and the Doctor discovers that the being controlling the Melkur is actually his old enemy, the Master, still little more than a withered almost-corpse on the edge of death, seeking to use the powers of Traken to, well, not be a withered almost-corpse, understandably...and presumably get rid of the crusty black cloak while he's at it. Thing must smell awful by now. Dun-dun-dun.
The Master and the Doctor do their usual dance of trying to beat each other, and while the Doctor does keep the Master from taking over the Source (some sort of Hive Mind for Trakenites), the Master has the last laugh. He hijacks Tremas's body, de-aging himself out of the Crispy Fried Master he has been and exulting in "a new body, at last".
Watch it here
- Ascended Extra: Nyssa was originally created just for this story, but the producers saw potential for her character as a companion and wrote her into the following story (though she was phased into it rather than joining the Doctor right away).
- The Bad Guy Wins: The Master may not get control of Traken but he gets what he wanted most: a new body.
- Clothes Make the Maniac: Kassia is turned evil by the necklace the Melkur gives her.
- Cool Chair: The Keeper's throne.
- Crystal Spires and Togas: Traken.
- Death by Origin Story: Both of Nyssa's parents die as a result of the Master's machinations. Her stepmother Kassia, whom the Master manipulated, then killed once he had no further use for her, and her father Tremas, whose body he wears for the rest of the 1980s. As if that wasn't enough, her home planet is destroyed in the next story.
- Epiphanic Prison: Anyone who is evil and winds up on the planet becomes imprisoned in a stone form called a "melkur" by the anti-evil effects of the area (It Makes Sense in Context). "Melkur" is their word for a fly trapped in honey. To be freed, all you have to do is not be evil anymore. Once your heart changes, you're free. Naturally, The Master had been stuck for a good long while. Not. The Melkur is really his TARDIS, whose chameleon circuit works just fine. He's been free and able to orchestrate the evilness that had been going on all along. He probably avoided actually becoming a melkur by staying inside and moving it wherever he needed to be, as the Melkur was sometimes seen to move.
- Evil Redhead: Kassia, after her Face–Heel Turn.
- Eye Beams
- Flowers of Romance: Kassia became gradually infatuated when she was a child to the Melkur (giving flowers to the humanoid). However, the Melkur turned out to be The Master, who manipulated her into killing and manipulating others in order to achieve the Keepership.
- Grand Theft Me: Tremas's body gets hijacked by the Master.
- Hijacked by Ganon: Contrary to the beliefs of some fans, people who watched this story unspoiled do claim to have been surprised when the Master was revealed.
- It Was Here, I Swear!: When the Trakenites go to where the TARDIS landed to verify the Doctor's story, it's vanished.
- Just One Second Out of Sync: How the Melkur hides the TARDIS from the Doctor and the Trakenites.
- Karmic Death: Kassia ends up selling out her entire society to the Melkur due to a combination of Love Makes You Evil and Love Makes You Stupid. Once she stops being useful, she gets what she so richly deserves.
- Meaningful Name: Tremas. His body is taken over by the Master, whose title is a Significant Anagram of "Tremas". The Master frequently used aliases related to his name — Reverend Magister, Mr. Seta, Colonel Masters, Sir Giles Estram — but usually he had a chance to pick them himself.
- Love Makes You Evil: Kassia is tricked into serving the Melkur because she doesn't want to lose Tremas.
- Living Statue: The Melkur turns out to be the Master's TARDIS, with the Master stuck inside.
- Nightmare Face: The Master. He gets a new one at the end.
- The Nth Doctor: Geoffrey Beevers takes over as the Master, only for Anthony Ainley to take over at the story's end.
- Not Using the "Z" Word: The Master is only called by his name once in the story, and neither actor who plays him is credited as such.
- Sealed Evil in a Can
- Significant Anagram: Averted, unusually enough for Doctor Who. "Tremas" is an obvious enough anagram for "Master", leading the viewer to think he's a baddie—but he is a good man and a staunch ally of the Doctor throughout. Not shockingly, the name of "Tremas" was demanded by producer John Nathan-Turner because actor Anthony Ainley, who played the character, would be playing the Master soon enough. Reflecting back on it, most of the cast think that it was a pretty silly reason for the name.
- Unfit for Greatness: Subverted. Luvic has this opinion of himself and so rules himself out for the Keepership. When the post needs filling in an emergency, though, he finds his fitness.
- Wicked Stepmother: Kassia has shades of this.
- The X of Y
- You Have Failed Me: Melkur has Neman executed for his chronic inability to keep the Doctor imprisoned.
- Your Princess Is in Another Castle: At the end, the Master has been defeated and possibly killed and the Doctor has left. Then the Master reappears, kills Tremas and gains a new body, and it turns out to be the first part of a trilogy.