[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/masque_1844.jpg]]
[[caption-width-right:350:TOUCHDOOOOOOOOOWN!]]
->'''The Doctor''': Humans have got such limited little minds. I don't know why I like you so much.
->'''Sarah Jane Smith''': Because you have such good taste.
->'''The Doctor''': That's true. That's very true.

The TARDIS pitches up in Renaissance Italy (well alright, in [[UsefulNotes/{{Portmeirion}} Portmeirion]]), having had an encounter en route with a fizzy ball of energy that the Doctor calls the Mandragora Helix. He hasn't realised that he's brought it with him (since he rather stupidly left the door open), and when it escapes from the TARDIS and begins taking over or killing the locals, he follows.

The energy finds an underground temple and takes over the Cult of Demnos, which is led by court astronomer Hieronymous. It instructs them to prepare the way for Mandragora's full appearance. However, the astronomer himself is a pawn in political intrigues between Count Federico and his nephew, Duke Giuliano. Giuliano has just acceded to the Duchy of San Martino (after the last Duke rather mysteriously came down with a case of death) and a grand Masque will be held in honour. Giuliano and his [[HoYay companion]] Marco hope to prove that Hieronymous is a charlatan. Meanwhile, the Doctor is sidetracked by almost being executed and steals a few horses, and Sarah Jane is kidnapped and hypnotised by the cult.

During the dance, the Cult of Demnos attack the court and kill many, while Hieronymous -- now unified with the Mandragora Helix -- confronts the Doctor in the underground temple. Attempting to blast the Doctor with pure Mandragora energy, he is foiled when the Doctor reveals he has earthed both himself (with some iron armor) and the altar: the energy is drained off and the Earth is safe.

Also, Sarah becomes the first companion to question how come she can understand and be understood when the locals must all be speaking Italian. The Doctor says it's a "Time Lord gift", and it's later explained that it's done telepathically via the TARDIS.

!!Tropes:

* {{Astrologer}}: Hieronymous.
* BeardOfEvil: Hieronymous has a glorious beard.
* BlackCloak: The cultists wear these, but Hieronymous opts for the purple pimp cloak and gold bling medallion seen in the image above.
* TheBlank: Hieronymous, after absorbing the Mandragora Helix.
* BloodlessCarnage: Giuliano and Federico both get wounded in swordfights, but not a drop of blood in sight. In Giuliano's case the Doctor even says he has to bandage him to stop the bleeding.
* BlueBlood
* BoundAndGagged: Sarah Jane.
* BrainwashedAndCrazy: Sarah Jane.
* CaliforniaDoubling: Filmed in UsefulNotes/{{Portmeirion}}, Wales (like many other TV period dramas of the time), which makes this episode rather distracting for fans of ''ThePrisoner''.
* ChosenOne
* ColdBloodedTorture: But Marco still refuses to falsely confess.
* CoolMask: The cultists' gilded masks, especially the one Hieronymous wears.
* {{Cult}}
* DamselInDistress
* DangerouslyGenreSavvy: Federico quickly recognises the Doctor as a troublemaker and orders his execution.
* DragonWithAnAgenda: Hieronymous.
* DramaticUnmask: Federico gets quite a shock when he rips off Hieronymous' mask.
* EnergyBeings: The Mandragora Helix.
* EvilIsHammy: Oh, Hieronymous...
* EvilLaugh
* EvilUncle: Count Federico.
* HeterosexualLifePartners: Giuliano and Marco, although the "heterosexual" is seriously debatable.
* HumanSacrifice
* LockedInTheDungeon: First Marco is locked in the dungeon, later on the Doctor, Sarah Jane and Giuliano join in on the fun.
* MalevolentMaskedMen: the Cult of Demnos.
* MasqueradeBall: The [[MalevolentMaskedMen Cult of Demnos]] takes advantage of Giuliano's swanky ascension party to conceal themselves in Part 4.
* TheMole
* NotHerself: The Doctor knows Sarah has been taken over... because she's the first companion to ask how come she can understand people who speak other languages. Err, right...
** It makes sense. Sarah had been travelling with the Doctor for so long, if she was going to ask how she can understand people who speak other languages, she would have done it long before now.
* OffWithHisHead: The Doctor finds himself with his neck on the executioner's block in one cliffhanger.
* ReasonableAuthorityFigure
* TheRenaissance: Takes place in the late 15th Century.
* ReligionOfEvil: The Cult of Demnos.
* RoyallyScrewedUp: Count Federico. He has Giuliano's father killed to bring him one step closer to becoming the Duke of San Martino, but ends up being unable to have Giuliano assassinated.
* SecretCircleOfSecrets
* SequelHook: The story ends with the Doctor revealing that the astronomical alignment that allowed Mandragora to attack Earth will recur towards the end of the twentieth century. This was not pursued in TV continuity. (The ''[[Series/TheSarahJaneAdventures Sarah Jane Adventures]]'' story "Secrets of the Stars" was originally conceived as picking up on this, but the writers decided that it would be too old and obscure a reference even for the Whoniverse, and went for a SuspiciouslySimilarSubstitute instead.)
** In the Big Finish audio drama range Sarah Jane Smith (which featured Sarah but not the Doctor), it is revealed that Giuliano wrote a book about his experiences of this adventure, called the book of tomorrows. This book became to basis of a cult (the Orbis Postramo or the Orphans Of The Future) which became involved in Sarah's life in the present day in the second series.
** The Mandragora Helix also appears in the Expanded Universe novel Beautiful Chaos with the Tenth Doctor and Donna in 2009 London.
* SmugSnake: Both Federico and Hieronymous are arrogant would-be tyrants, but both are also convinced that they have more control over the situation than they do. Federico is so consumed with his schemes to overthrow his nephew that he ignores the real threat (the Cult of Demnos and the Helix) until it's too late; similarly, Hieronymous is a deluded fraud trapped by superstition and seduced by dreams of glory who ends up utterly consumed by the Helix.
* SwirlyEnergyThingy: The Helix itself.
* SwordFight: The Doctor merrily joins in.
* TapOnTheHead
* TargetedHumanSacrifice: Sarah Jane, who was apparently predicted to turn up.
* TranslatorMicrobes
* UnwillingSuspension
* WhenThePlanetsAlign
* TheWisePrince: Giuliano.
* TheXOfY
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