The TARDIS parks Ace and the Seventh Doctor in a POW camp in UsefulNotes/WorldWarII Germany. Ace is understandably pissed off when they enter what looks like a giant classic mansion, since that's [[Recap/DoctorWhoS26E2GhostLight usually]] a sign of the Doctor wanting to manipulate her with some kind of masterplan. Fortunately, the Doctor has no idea where they are. Unfortunately, they immediately get themselves captured by Nazis, separated, and imprisoned as Prominente: ally spies.
The Doctor baffles everyone with his two hearts, and plays some spoons on the Nazis. Ace, meanwhile, attracts a very nasty character named Feldwebel Kurtz (played by Creator/DavidTennant!), who tries to get her naked and at his mercy as quickly as possible. He's also very confused by her portable CD player. Luckily for Ace, these are the kind of realistic Nazis who adhere to the Geneva Conventions and don't particularly care for Hitler's master race theory, so she's treated like an actual human being and Kurtz is reprimanded for perving on her.
While Ace conspires with the other English prisoners (she's sure they can escape -- she's played the board game and everything), the Doctor meets a very, ''very'' nasty Nazi officer named Elizabeth Klein, who tries to steal his TARDIS. He lets her know that no matter how she threatens him, and no matter what her designs on his TARDIS are, she is ''not'' in charge and she has ''no'' idea what she's messing with.
Klein reveals herself to be from the 1960's -- she's travelled to 1944 by TARDIS. The Doctor's future TARDIS. But when she tries to find it again, it's disappeared from where she parked it, and she's stranded in 1944... unless she can appropriate the TARDIS that's now in Nazi hands. The Doctor realises that Klein has ''absolutely no idea'' what she's doing, and that her plan to bring the TARDIS into Nazi hands is pointless, since it would have had to be in Nazi hands in the first place for her to get to 1944. Klein counters that with the Doctor now stranded and Ace soon to be dead in a failed escape attempt, the Nazis will have ''plenty'' of time to interrogate the Doctor, enabling the Holy German Reich to control the TARDIS in the 1960's. Because in Klein's timeline, the Nazis won the war.
The Doctor really tries his best to explain to Klein that her timeline is not the regular one. She has some trouble coping with this, and accuses the Doctor of being no worse than the Nazis if his idea of a "normal" universe includes erasing the lives of everyone in her own timeline.
Meanwhile, Ace doesn't actually die in her escape attempt, because Kurtz wants to suck up to Klein and decides to let Ace live for that reason. This means that Klein's time travel is already starting to unravel the Web of Time. When Klein reveals that she was only able to pilot the TARDIS with the help of her friend Johann Schmidt, the Doctor finally realises what happened: after being shot and killed by the Nazis in the future, he would have regenerated into the Eighth Doctor, befriended Klein for years, and eventually convinced her to steal the TARDIS from Nazi headquarters and travel back in time. Eight also programmed the TARDIS to vworp off as soon as Klein landed, leaving her stuck. All along, Klein was an unwitting pawn, whose only purpose was to loop back in on her own paradox (which Eight couldn't have done without very seriously upsetting time itself) and undo everything that happened, including the Doctor's own regeneration.
And if the Nazis were never able to control the TARDIS without Eight's help, something else must have led them to win the war... [[TimelineAlteringMacGuffin Ace's CD player. Its technology gave the Nazis ''lasers']]', which naturally led to the discovery of nuclear power well before its time. Ace is ''not'' happy with the realisation that her carelessness enabled a global holocaust.
In the end, Klein manages to do what no one else in the story had so far managed: escape from Colditz castle. (Although this is far from the last the Doctor will see of her). Hauptmann Julius Schäfer -- an ally sympathiser -- betrays the Nazis and helps the Doctor and Ace escape as well. Kurtz dies a ''very'' messy death when the TARDIS vworps off with him stuck in her doors.
Ace, thoroughly traumatised by the whole experience, decides that it's time for her to grow up a bit. She asks the Doctor to start calling her Dorothy.
* TheAlcatraz: Colditz, based on it's RealLife counterpart.
* AffectionatePickpocket: The Doctor
* ALittleSomethingWeCallRockAndRoll: The Nazis find Ace's portable CD player (that she aquired in [[Recap/BigFinishDoctorWho005TheFearmonger "The Fearmonger"]]). It doesn't end well.
* AlternateHistory: Elizabeth Klein is from an alternate world where the Nazis won UsefulNotes/WW2.
* TheAtoner: The alternate Eighth Doctor spends pretty much his entire life trying to correct time to its proper course. It's up to interpretation if he's trying to attone for the corruption of the timeline, or if he's trying to prevent Ace's untimely death by meddling. Probably both.
* TheBaroness: Madame Klein.
* BatmanGambit: By the Eighth Doctor.
* BavarianFireDrill: E. Klein]] pulls this.
* BreakTheCutie: As the next episode shows, Ace is ''very heavily'' traumatised by this episode's ending.
* BreakThemByTalking: The Doctor pulls this a few times in the story.
* ButterflyOfDoom: [[InvokedTrope Invoked by the Doctor]].
* [[ByTheBookCop By The Book Nazi]]: Kurtz. Schäfer as well, but to a lesser extent.
* TheChessmaster: For a change, ''not'' the Seventh Doctor. The Eighth Doctor, "Johann", spends over 10 years just trying to find a person whom he can potentially convince to step back in time and correct things. He then spends the next several months slowly convincing her to go back in time to "save" the Doctor from the original timeline on the pretence of using him to explain how the TARDIS works. He's good.
* ContinuityNod: A brief one to "Ghost Light" at the beginning, and Ace got that walkman in "The Fearmonger".
** The Doctor [[Recap/DoctorWhoS25E1RemembranceOfTheDaleks once again]] asks Ace to use some explosives which she [[SincerityMode explicitly stated]] she didn't have in her rucksack.
* CrowdChant: "Let her go! Let her go!"
* DayOfTheJackboot: The Doctor says this is one of TheOldestOnesInTheBook.
* DirtyCoward: Tim betrays Ace.
* FakingTheDead: The Doctor. Done quite easily, considering he regenerated into another body.
* FishOutOfTemporalWater: Dr. Klein.
* GodwinsLaw: Klein tries to convince the Doctor he's NotSoDifferent from the Nazis. He doesn't bother to listen.
* GodwinsLawOfTimeTravel: Klein comes from an alternate future where the Nazis win World War II.
* HeroicSacrifice: The Eighth Doctor gets at least shot trying to get Klein into the TARDIS, not to mention essentially erased from history once his plan works.]]
* IdiotBall: Ace carries it big time by refusing to take the Nazis seriously, constantly taunting them and assuming it will be easy to escape.
* ImperialStormtrooperMarksmanshipAcademy: Elizabeth.
* LeaningOnTheFourthWall: The Doctor briefly hums something that sound suspiciously like the ''Series/DoctorWho'' theme song.
* MasterRace: Klein has blue eyes and blonde hair, and has German parents.
* MyCountryRightOrWrong: Kurtz, Schäfer. Klein is just a [[ItsAllAboutMe bitch]].
* NotSoDifferent: Klein thinks that the Doctor restoring the timeline is just as bad as what she's doing.
* OfficerAndAGentleman: Schäfer.
* OhCrap: The Doctor as soon as he realizes where they are.
* OnlyAFleshWound: The Doctor after he gets shot.
* OohMeAccentsSlipping: David Tennant is mostly flawless with his German accent, but slips into Scottish for a brief moment.
* PortalCut: Kurtz attempts to shoot at the Doctor and Ace while half-in/half-out of the TARDIS. His gunfire causes an emergency dematerialisation. The results are... unpleasant.
* ReasonableAuthorityFigure: Schäfer
* SetRightWhatOnceWentWrong: Klein, inadvertently.
* SherlockScan: The Doctor deduces what Klein really is fairly quickly.
* StupidJetpackHitler: Klein's world springs from the successful Nazi reverse-engineering of Ace's CD Walkman and the capture of the TARDIS.
* ThatManIsDead: This episode causes Ace to drop "Ace" and go back to Dorothy [=McShane=]. For a while.
* ThoseWackyNazis: Generally avoided, as the Nazis are portrayed in a very realistic light. Until Klein shows up, that is.
* TimelineAlteringMacGuffin: Ace's CD player gives the Nazis laser technology.
* TimeTravelTenseTrouble: This story allows World War II to take its course as we know it.
* UnwittingPawn: Klein for the Doctor.
* UtopiaJustifiesTheMeans: Completely DENIED by the Doctor.