The Doctor returns to the 2090s to pick up Beth, who is now seventeen. She asks that they go see a certain play in 1988: ''Theatre/TheSecretOfSherlockHolmes'', starring Creator/JeremyBrett and Edward Hardwicke. Holmes is less than impressed with the titular secret and his doppelganger. Torchwood kidnaps Jeremy, mistaking him for Holmes, and captures the TARDIS. The Doctor & Co make plans to rescue both, but nothing goes according to plan, and Watson realizes that his best friend is [[TookALevelInJerkass changing dramatically]].
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!!This Episode Provides Examples of:

* AlcoholInducedIdiocy: Holmes gets himself and Jeremy drunk and tries to pick a fight with the older man until poor Jeremy has enough and stalks off. [[spoiler:And directly into a kidnapping.]] Holmes does nothing ''directly'' stupid, but he does act like a drunken idiot.
* AscendedExtra: Sally Sparrow and Beth Lestrade.
* BenevolentBoss: Bernice Partington is really just doing her job. May overlap with ReasonableAuthorityFigure.
* BewareTheNiceOnes: All over the place, but nowhere more ''startling'' than when the ever-affectionate-towards-his-Companions ''Tenth Doctor'' angrily '''manhandles''' a drunken Holmes.
* ContinuityNod: The episode is one big nod to canonical and internal continuity, with the events of "Army of Ghosts" and "Doomsday" playing a not-so-small role in the Doctor's interactions with Torchwood.
** Also, there's quite a ''big'' throwback to one brief moment in "Rose," when Clive tells Rose that the Doctor kept a family from going aboard the ''Titanic''.
* CoolBigSis: Sally's interactions with Beth.
* EveryoneCanSeeIt: Played with, as Beth is very aware of her feelings for Holmes and quickly confides in Sally. Ironically, the Great Detective seems to be the only one who ''can't'' see that Beth is in love with him -- indeed, is quite uncertain of his own feelings for her.
** Even ''Jeremy'' implies that he gets it while Holmes doesn't.
* FirstNameBasis: Beth quickly gets into this with Holmes (despite having done it once already).
* {{Foreshadowing}}:
** The play. Holmes and Watson have no idea just how ''real'' the "alternate interpretation" of their lives is about to get...
** Beth deciding to commit a HeroicSuicide.
** And then Holmes following up on that by threatening to kill himself.
* FormulaBreakingEpisode: Up 'til now, the series, with the exception of the first episode, has been following [=DW's=] Season 3. ''This'' episode, however, is not tied with ''any'' episode from that season and, in fact, is loosely InspiredBy [[Recap/DoctorWhoS31E10VincentAndTheDoctor "Vincent and the Doctor"]].
* FriendVersusLover: Averted, even though Holmes fits the GreenEyedMonster to a tee -- Sally doesn't want to come between her boyfriend and his [=BFF=].
* GreenEyedMonster: Mm, these quotes (see InVinoVeritas below)...
-->"Poor girl’s barely outta high school--you should be ashamed o’ y’self..."
-->"Can’ ''think'' wha’ Beth sees in that guy, he’s insuff... suffer... he’s a jerk."
* HeroicSacrifice: Beth attempts this by holding herself hostage for the sake of the Doctor's freedom (and sanity).
* HistoricalHeroUpgrade: Creator/JeremyBrett, in a mild version of the trope.
* InVinoVeritas: When Holmes gets himself drunk, the things he says about Beth to Jeremy and the Doctor are ''very'' telling...
* JerkWithAHeartOfGold: Holmes... we think. It's disturbingly hard to tell in this episode -- he's certainly a far cry from the compassionate man he was a couple of episodes ago. He's not entirely a jerk in this story, but, dang, he does have some extremely cringe-worthy moments.
* JumpedAtTheCall: Beth, pretty literally (you can hardly blame her -- she's waited two years, after all).
* MistakenIdentity: [[TheWoobie Poor Jeremy]]. A handful of Torchwood goons who can't tell the difference between him and Holmes kidnap him, thinking that he ''is'' Holmes. Fortunately for Jeremy, the Director instantly recognizes him and rectifies the situation.
* NiceJobBreakingItHero: If Holmes hadn't gotten himself and Jeremy drunk, Jeremy might not have been kidnapped.
* ObliviousToLove: Holmes, as explained above.
* OfficialCouple: John ''and'' Sally Watson.
* OneHeadTaller: Averted with both girls, who are each exactly three inches shorter than their men: Sally is 5'7" to Watson's 5'10", and Beth is 5'11" to Holmes's 6'2".
* SexyDiscretionShot: The closing scene -- the Watsons are shown just getting started on their wedding night, with Sally assuring her chivalrous husband that, yes, she's very ready for this.
* SheIsAllGrownUp: Holmes ''does'' notice.
** SheCleansUpNicely: With grown-up '80s hair and makeup, Beth ''definitely'' passes for early twenties, and it stuns Holmes speechless. (Of ''course'', Beth misses the stare.)
* ShipperOnDeck: Torchwood, of all people, are entirely content to allow Sally to become the new Mrs. Watson.
* TookALevelInJerkass: Holmes. ''Ouch''.
* WillTheyOrWontThey: Are Holmes and Beth the incoming AlphaCouple or not...
* YoungerThanTheyLook: Jeremy is surprised to find out that Beth is only seventeen. Likewise, the Doctor tells her that taking away her ponytail and putting on some makeup would definitely age her appearance.
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