->''The Germans seem to know every move we make. I had a letter from Jerry yesterday. It said, "Isn't it about time you changed your shirts, Walrus-Face?"''

In the aftermath of an artillery bombing on their trench, George winds up in the field hospital (much to Blackadder's annoyance). Soon after, Blackadder finds himself called to Melchett's office: a German spy is giving away every one of their battle plans (much to Blackadder's surprise, who didn't think they actually ''had'' battle plans). According to their intel, the leak is coming out of the field hospital and Blackadder is given three weeks to root them out.

Taking up in the hospital, Blackadder finds himself irked by the overly soppy Nurse Mary, though he [[BreakTheHaughty does get some fun out of new patient Darling]] (who Melchett shot in the foot to give him a cover story for keeping an eye on Blackadder). However, Blackadder discovers to his surprise Nurse Mary's soppy manner is just an act for the benefit of the patients, and she's every bit as cynical and jaded as him, and they hit it off (and have it off) as Blackadder continues to track down the spy.
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!!Tropes
* AccentRelapse: Inverted by Smith who picked up a strong accent while undercover in Germany and hasn't been able to drop it after returning.
* AcademiaElitism: As part of Blackadder's investigation, he asks Nurse Mary if her boyfriend went to any of the great British colleges, like Oxford, Cambridge, or Hull. When he later formally accuses her, he points out his SecretTestOfCharacter: that only two of those colleges are "great universities"--to which General Melchett chimes in that [[ComicallyMissingThePoint "Oxford's a complete dump."]][[note]]Blackadder's line was meant to be at Hull University's expense (being rather more obscure than Oxford or Cambridge, [[ArtisticLicenseHistory besides the fact it didn't exist yet in 1917]]), but Creator/RowanAtkinson (Blackadder) is an Oxford grad and Creator/StephenFry (Melchett) went to its traditional rival Cambridge, so Fry got in a jab at his co-star's alma mater as a BaitAndSwitch.[[/note]]
* ActorAllusion:
** Melchett referring to Oxford University as "a complete dump" is a nod to the fact that Creator/StephenFry went to Cambridge while Creator/RowanAtkinson went to Oxford.
** An in-universe example. Creator/MirandaRichardson plays Nurse Mary, a seemingly fluffy and silly woman whose real personality is, well, rather different -- much like Amy the Highwayman in "[[Recap/BlackadderS3E5AmyAndAmiability Amy and Amiability]]".
* AmbiguousEnding: Nurse Mary is last seen being led away by Baldrick to a firing squad, following which Blackadder finds out the truth (that she's not a spy, the Germans were getting all of their information from George's letters to his uncle in Munich) and rushes out to stop the execution. We never find out if he was successful — but as he's back in the trenches as of the start of the next episode, his appointment as head of Operation Winkle (which would have necessitated a move away from the front line) was evidently rescinded.
* AnalogyBackfire: For Darling, when trying to convince Blackadder that he is not a spy:
-->'''Darling:''' I'm as British as Queen Victoria!\\
'''Blackadder:''' So, your father's German, you're half-German and you married a German? [[note]] In fact, although Queen Victoria was indeed half-German, it was her ''mother'', Princess Victoria of Saxe-Coburg-Saalfeld, who was German. Her father, Prince Edward (the Duke of Kent and Strathearn), was English by birth, upbringing and culture -- although he was in fact a fourth-generation German immigrant whose father (George III) was also the King of Hanover, while his mother (Queen Charlotte) was a German princess from Mecklenburg. [[/note]]
* ArtisticLicenseHistory: Blackadder mentions three of the great universities: Oxford, Cambridge, and Hull. The University of Hull was founded as a college in 1927 and became a university in 1954. While Blackadder had intended this as a trick question, Melchett's comment about Oxford being a complete dump (thereby implying that Hull has already been founded) plays the trope straight. Then again, Melchett is insane, so...
* AskAStupidQuestion: Baldrick's plan to find a German spy would be to walk up to a suspect and ask if they are a German spy. In the final scene of the episode, Darling asks his main suspect if he's a spy. The suspect openly admits it. The stupid part is that he's a ''British'' spy, an outcome which Darling had failed to consider.
* AssholeVictim: Given Mary's disdain for most of her patients and her attempts to frame Darling as a spy ForTheLulz, being convicted herself almost seems karmic, if still incorrect.
* ChekhovsGun: George's letters to his uncle are mentioned early on, and we hear him reading out part of one of them which describes the aftermath of his getting wounded.
* ComicallyMissingThePoint:
-->'''George''': What about you, Smithie, seen any German spies around here?\\
'''Smith''': ''[In thick German accent]'' Nein!\\
'''George''': ''Nine?'' Well, Cap's got his work cut out for him, then!
* CreatorInJoke: Blackadder says that he tricked Nurse Mary by naming three great universities (Oxford, Cambridge and Hull), when in fact only two of them are great. Melchett responds "Quite -- Oxford's a complete dump!" Creator/RowanAtkinson attended Oxford[[note]] As did Tim [=McInnerny=] (Darling) and series writer Richard Curtis[[/note]], while Stephen Fry attended Cambridge [[note]] As did Hugh Laurie (George) and series producer John Lloyd[[/note]], the two universities having a centuries-long rivalry.
* DisproportionateRetribution: Melchett threatens to have Blackadder shot for being rude to a lady.
* DoYouWantToCopulate: Nurse Mary is pretty blunt.
-->'''Nurse Mary:''' Edmund, when this war is over, do you think we might get to know each other a little better?\\
'''Blackadder:''' Yes, why not? When this madness is finished, perhaps we could go cycling together, take a trip down to the Old Swan at Henley and go for a walk in the woods.\\
'''Nurse Mary:''' Yes, or we could just do it right now on the desk.\\
'''Blackadder:''' ''[looks at the desk]'' Yeah, okay.
* EntertaininglyWrong: Blackadder deducing that Nurse Mary is the German spy. His reasoning is perfectly sound and the suspect had already admitted to using ObfuscatingStupidity in front of others. Alas, the true 'spy' was someone else altogether. Mary was completely innocent, while George was unknowingly giving information to the enemy all along, by way of his not-properly-censored letters to his German uncle.
* EvenEvilHasStandards: Before being convicted, Mary attempts to frame Darling as a spy ForTheLulz, something even Blackadder, despite his tormenting of Darling earlier, doesn't have the heart to go through with. He falsely convicts Mary, but at least under the mistaken assumption she is genuinely a spy. In turn he is genuinely horrified when he realises he got it wrong and is last seen frantically trying to halt her execution in time.
* HiddenDisdainReveal: Blackadder discovers Mary's true self when she snides about George's idiocy in closed quarters.
* IdiotBall: George utterly outdoes himself here, as it ''never once'' occurs to him that maybe detailing top-secret information in letters to his uncle ''who lives in Germany'' (where, of course, all letters are intercepted and read by the authorities) is a bad idea.
* ItsAllAboutMe: Edmund's brutally honest about this.
-->'''Nurse Mary''': Tell me, Edmund, do you have someone special in your life?\\
'''Blackadder''': Well yes, as a matter of fact, I do.\\
'''Nurse Mary''': Who?\\
'''Blackadder''': Me.\\
'''Nurse Mary''': No, I mean someone you love and cherish and want to keep safe from all the horror and the hurt.\\
'''Blackadder''': Um... still me, really.
* MyGirlBackHome: Darling reveals he has a girlfriend, Doris, as part of his attempt to convince Blackadder that he's not the German spy. Blackadder himself is an aversion, when Mary asks he reveals that he doesn't have anyone waiting for him in England.
-->'''Blackadder''': I've always been a soldier, [[MarriedToTheJob married to the Army]]. Book of King's Regulations is my mistress...[[NotSoAboveItAll possibly with a Harrods lingerie catalogue discreetly tucked between the pages]].\\
'''Mary''': [[FriendsWithBenefits And no casual girlfriends]]?\\
'''Blackadder''': Skirt? If only! When I joined up, we were still fighting colonial wars. [[TakeThat If you saw someone in a skirt, you shot him and nicked his country!]]
* NobodysThatDumb: Why Blackadder didn't even consider Smith to be the spy for a second and [[LampshadeHanging lampshades]] to Darling when he's convinced the man is a spy (he is, but a British one). Not even the Germans would be stupid enough to field a spy with a strong German accent.
* ObfuscatingStupidity: Nurse Mary uses a mild version of this. She calls it "my fluffy-bunny act".
* OfficerAndAGentleman: Melchett, of all people.
-->'''Melchett''': Explain yourself, Blackadder, before I have you shot for being rude to a lady!
* PlotArmor: After George got hurt, Blackadder called him a lucky bastard, then climbed to the top of the trench and yelled "What about me, Fritz?! I'm over here!" The only reasons he didn't get shot is probably a) the Germans were taking cover from artillery shells, b) they didn't think that one guy was worth using the ammunition, c) they were so confused by the fact a Brit ''wanted'' to get hit they couldn't decide what to do, or d) none of them were ''that'' decent a shot.
* PoorMansPorn: Apparently, Blackadder uses a Harrods lingerie catalogue.
* RedHerringMole: "Mister Smith", who speaks with a HEAVY German accent. Also leads to this:
-->'''[[{{Keet}} George]]:''' You haven't seen any suspicious-looking characters that might be German spies, have you Smitty?\\
'''Smith:''' Nein!\\
'''[[TheDitz George]]:''' [[ComicallyMissingThePoint NINE!?]] [[SugarWiki/FunnyMoments Cap's got his work cut out for him, then!]]
** Of course, Smith does turn out to be a spy ... a '''British''' spy, who developed "[[{{Understatement}} a teensy bit of an accent]]" from working undercover in Germany for so long; Blackadder realises that the Germans aren't dumb enough to send such an obvious spy, but Darling doesn't, tries to arrest Smith, and ends up being chewed out by Melchett for his incompetence.
* ShoutOut: There are references in Nurse Fletcher Brown's name towards ''Film/OneFlewOverTheCuckoosNest'', the actress Creator/LouiseFletcher who plays Nurse Ratchett and Alonzo Brown who plays Dr Miller.
* SomethingThatBeginsWithBoring: The game of I Spy at the beginning between Baldrick and George driving Blackadder to distraction.
* StatusQuoIsGod: The end of the episode sees Blackadder given the permanent role of head of Operation Winkle meaning he'll be leaving the trench potentially for good. However, since he'd got the wrong person sentenced to death and ended the episode racing to inform the General before Captain Darling, we can assume that the job offer got rescinded as the next episode sees him back in the trench. George somehow escapes punishment for being the actual spy (albeit unwittingly and unknowingly),
* UncertainDoom: Whether Blackadder and Darling vindicate Mary before she meets her end by firing squad is never revealed. Given that Baldrick was the one leading her to the firing squad, who knows what might have happened.
* WhamLine: From George...
-->Oh well, lots of exciting stuff to put into my next letter to my Uncle Hermann in Munich.
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