[[caption-width-right:345: The Keeper, sitting on his [[CoolChair magical moving chair.]]]]
->''"They say the atmosphere there was so full of goodness that evil just shriveled up and died. Maybe that's why I never went there."''
-->-- '''The Doctor'''
Welcome to Traken, a world so perfect that [[SugarBowl 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 Main/TheMole 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 OmniscientCouncilOfVagueness, 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 [[ArchEnemy old enemy]], the Master, still [[Recap/DoctorWhoS14E3TheDeadlyAssassin 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 HiveMind for Trakenites), the Master has the [[EvilLaugh last laugh]]. He [[GrandTheftMe 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 [[http://www.dailymotion.com/playlist/x1019s_tardismedia_the-keeper-of-traken here]]
* AscendedExtra: 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).
* TheBadGuyWins: The Master may not get control of Traken but he gets what he wanted most: a new body.
* ClothesMakeTheManiac: Kassia is turned evil by the necklace the Melkur gives her.
* CoolChair: The Keeper's throne.
* CrystalSpiresAndTogas: Traken.
* EvilRedhead: Kassia, after her FaceHeelTurn.
* FlowersOfRomance: 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 [[LoveMakesYouEvil killing and manipulating others]] in order to achieve the Keepership.
* GrandTheftMe: Tremas's body gets hijacked by the Master.
* HijackedByGanon: Contrary to the beliefs of some fans, people who watched this story unspoiled do claim to have been surprised when the Master was revealed.
* ItWasHereISwear: When the Trakenites go to where the TARDIS landed to verify the Doctor's story, it's vanished.
* JustOneSecondOutOfSync: How the Melkur hides the TARDIS from the Doctor and the Trakenites.
* KarmicDeath: Kassia ends up selling out her entire society to the Melkur due to a combination of LoveMakesYouEvil and LoveMakesYouStupid. Once [[YouHaveOutlivedYourUsefulness she stops being useful]], she gets what she so richly deserves.
* LoveMakesYouEvil: Kassia is tricked into serving the Melkur because she doesn't want to lose Tremas.
* TheNthDoctor: Geoffrey Beevers takes over as the Master, only for Anthony Ainley to take over at the story's end.
* NotUsingTheZWord: The Master is only called by his name once in the story, and neither actor who plays him is credited as such.
* SignificantAnagram: 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" [[ExecutiveMeddling 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.
* WickedStepmother: Kassia has shades of this.
* YouHaveFailedMe: Melkur has Neman executed for his chronic inability to keep the Doctor imprisoned.