Written by Jacqueline Rayner, "The Marian Conspiracy" was the first Doctor Who audio story to introduce an original companion, [[CoolOldLady Dr. Evelyn Smythe]]. The story itself is something of a throwback, dealing with purely historical events and figures and having no alien threat or major science-fictional conceit, something that had been unseen (or rather unheard, as it is) since the era of the First Doctor (barring [[Recap/DoctorWhoS4E4TheHighlanders two]] [[Recap/DoctorWhoS19E5BlackOrchid exceptions]]). The success of this story led Creator/BigFinish to bring back the occasional historical adventure, too.
Tracking a nexus point in time, the Sixth Doctor meets Dr Evelyn Smythe, a snarky 55-year-old history professor whose own history seems to be rapidly vanishing. The Doctor must travel back to Tudor times to stabilise the nexus and save Evelyn's life. Six (mercifully) decides to dress up in ruffles and velvet for the occasion, but Evelyn insists on taking her cardigan and handbag along and walking straight into the nearest pub while the Doctor goes to find the Queen. He's immediately mistaken for her personal physician, and is extremely surprised to find that the TARDIS has landed them three years early and Bloody Mary is still on the throne. Also, she's pregnant, which is of course downright impossible.

Evelyn, meanwhile, is mistaken for a member of LaResistance when she proclaims her loyalty to Elisabeth I. She's taken along to LaResistance's hideout and introduced to its members. Despite being careful with her words (and making them all some nice hot cocoa), she draws the attention of Reverend Thomas, who's trying to actually ''assassinate'' Mary for being Catholic -- but couldn't bring himself to it, because of her pregnancy. When Evelyn tells him that it must only be a phantom pregnancy, Thomas hastily revises his plans and begins plotting against everyone around him, trying to assassinate Mary as soon as possible.

Meanwhile, the Doctor has found a kindred soul in Bloody Mary, and manages to keep up excellent bedside manner for a good while. They discuss personal convictions, whether it's ever okay to commit mass murder on people who go against one's beliefs, and what it feels like to live with the blood of thousands on one's hands. The Doctor is very shaken by the conversation, but Mary -- a true WellIntentionedExtremist -- is quite happy with her decision to burn all Protestants at the stake. After all, it is only a sign of religious mercy, a way to save her beloved country. In the middle of all that, Queen Mary's handmaiden -- Sarah -- turns out to be unknowingly plotting to assassinate Mary, not knowing that Thomas would use her to actually ''murder'' the Queen. She and Thomas are married, but the Catholic law made their marriage invalid. They continued to be part of LaResistance in secret. And Sarah is pregnant... with Evelyn's ancestor.

Once the Doctor figures all that out (after a brief stint in the Tower), he convinces Mary to let Sarah be the handmaiden to the captive Queen Bess as punishment. He rescues what's left of LaResistance and moves them safely to Germany, telling them that English Catholicism will only last for three more years. There's nothing he can do to prevent Thomas' death, though, and both Six and Mary end the adventure very shaken by their experiences. Evelyn promises to bake Six some nice chocolate cake. She also announces that she'll be his new companion from now on, ignoring his protests about the matter.
* CallForward: The Doctor and Evelyn do eventually pop back to give Darwin some moral support. [[HeroicBSOD At a quite vital time, no less]].
* CareerVersusMan: Subverted: Evelyn chose attending an academic conference over observing her wedding anniversary, and actually divorced her husband to do so.
* CardboardPrison: The Tower. Lampshaded, but still not quite realistic. The ridiculousness of the escape became a RunningGag throughout Creator/BigFinish and got a bit more lampshading later on in "Project: Twilight", "Jubilee", "Colditz" and "Something Inside".
* ContinuityNod:
-->'''Evelyn''': I suppose all of that out there will be destroyed by the [[Recap/DoctorWhoS19E4TheVisitation Great Fire.]]
-->'''Sixth Doctor''': Uhh...yes.'' (uncomfortable sounds)''
** Evelyn has weak knees. The Doctor remarks that [[Recap/DoctorWhoS2E2TheDalekInvasionOfEarth it's usually the ankles]].
** Note how the Doctor doesn't offer himself to take the aspirin?
* ContrivedCoincidence: Out of all the people living in London at the time, Evelyn just happens to become involved with the two plotting to overthrow Mary and replace her with Elizabeth, simply by visiting the local pub.
* CoolOldLady: Doctor Evelyn Smythe -- so, so much.
* DatedHistory: The story is heavily dependent on the idea that Mary had a phantom pregnancy, which for a long time was the accepted story. In fact, these days most historians believe she either had [[https://en.wikipedia.org/wiki/Molar_pregnancy a molar pregnancy]] or maybe ovarian cancer.
* DeadpanSnarker: Evelyn has her glorious moments:
-->'''Sixth Doctor''': (While being held captive in the Tower of London) I might not want to mention [Lady Jane Gray] to the Queen, it might upset her.
-->'''Evelyn''': And we wouldn't want to upset the Queen, now would we? She might have us thrown in the tower!
* DrinkOrder: Evelyn carries hot chocolate mix. Her fondness of the drink returns later in other audio adventures.
* EstablishingCharacterMoment: Being her first story (and the first instance of a new audio companion), Evelyn gets hers when she, during her first trip in the TARDIS, doesn't let [[InsufferableGenius the Sixth Doctor]] talk over or boss her around while deflating any of his attempts to puff himself up with a self-serving story.
* ExpectingSomeoneTaller: Evelyn, when she first sees the TARDIS interior.
* FixFic: The Sixth Doctor spends most of the episode not wearing That Coat, gets a new companion who puts up with precisely zero of his ego-trips, shows off his excellent bedside manner and displays genuine kindness to Mary, and gets in a proper heartfelt monologue about his guilt issues.
* HardDrinkingPartyGirl: Evelyn regularly parties with her students, and recalls taking part in a gin drinking contest with them. (The gin won.)
* HistoricalDomainCharacter: Mary I of England.
* MoodDissonance: This is remarkably one of the more upbeat and light-hearted Doctor Who stories, but it deals entirely with the reign of terror under one of the most ruthless female rulers of modern history, Queen Mary I.
* NotThatKindOfDoctor: One of the first conversations The Sixth Doctor and Evelyn have is over their professions.
* NotSoDifferent: The Doctor and Mary I. He's very shaken once he realises that he's not in any position to lecture her.
* RippleEffectProofMemory: Evelyn remembers her ancestors and family tree, even though they've been erased from history and the Doctor doesn't know about them. The fact that the names she knows no longer appear in her records, and indeed vanish before her eyes, are what convinces her to believe what the Doctor is telling her.
* WellIntentionedExtremist: Mary I, who's portrayed in a very kind light while planning to commit mass murder on all "heretics".
** The Doctor has a quite powerful moment where he questions his own actions after meeting her.
--->What would you say if I were to tell you that I once destroyed an entire race, that I have led friends to their deaths and caused numerous wars? That my intervention has led to peaceful people taking up arms and good people having their faith or reason destroyed? Because I failed to act millions upon millions of people have been enslaved or killed? What if I had done all of those things but had always, always believed I was doing to the right thing?