Hello and welcome to "Why _ works (or doesn't)", where I go over in detail why an episode of the show succeeded or failed.  If you like or hate this, let me know and feel free to shout out suggestions for episodes that should go on the chopping block or praise podium. Today I'm analyzing Supernatural 12.12, "Stuck in the Middle (with you)".
What is a retcon?  TVTropes defines it as "Reframing past events to serve a current plot need."  The thing is, can you tell the difference between that and a revelation in the plot? Both are moments that come late in a story which change or alter the meaning of events, actions, and characters which happened earlier in that story. 
Fundamentally the difference is with the author.  A revelation is an idea the author had in mind before the the story began and always intended for the reader to learn (i.e. The plot twist in the movie The Sixth Sense.)  A retcon is something which the author did not come up with until after the story began and had never intended it at the start. (i.e. Comic books – any of them.)  One is planned, the other is improvised.
Ideally, retcons strive to be interpreted as revelations.  The best retcon ever pulled is the one the audience believes you had planned all along.  Conversely, the worst revelation ever executed is the one which convinces the audience that it is a clumsy retcon.
Of course when it comes to a long-form fiction like Supernatural, retcons are going to occur just as the nature of the medium.  Eric Kripke back during the original run often didn't know if he was going to get a third season, much less that stories would still be told into the twelfth so obviously there's been no plan for the show since his original run.  Yet the retcon in this episode is pulled off so masterfully, it's possible to pretend for a moment that there was.
We start with Crowley, who is back in full form after several seasons of aimlessness for his character.  At the time in season 6 (S6), we had never quite known exactly how Crowley became King of Hell.  Given the general story direction of S6 and "the power of souls", the implication to the audience was that Crowley had used his position as chief salesman to gather souls until he had more power than any other demon around.  It was hinted at with lines such as
Was a punk-ass crossroads demon. Now? King of hell. Believe me, I've got the mojo. -Crowley in "Family Matters" (S6e07)
My position isn't all that stable, ducky. Those souls would help, just like they'd help you. Besides, wouldn't you rather have me in charge down here? The devil you know...-Crowley "The Man Who Would Be King" (S6e20)
In this episode we learn that someone who had every right to be King of Hell handed it over to Crowley. Yes, it does seem odd that Crowley can become KoH just by saying "ok" without a kiss or even a handshake to seal the deal.  However it is a common fantasy staple that beings of sufficient power are able to "cause" things with casual conversation. So that moment does fit in story logic even if now some of the lines quoted above come off as a little wonky. (It does also make one wonder how the "Hell civil war" was supposed to work in S9 but, again, that was S9 so who cares.)
More importantly, Crowley functioned as a character this time. I had to laugh when Ramiel told Crowley, "they'll kill you next" since we all surely remember the time when the brothers had Crowley absolutely dead to rights and didn't kill him. (Thankfully he doesn't bring that up since it's best to forget about S9.) Instead we see him trying to save both the Winchesters and Ramiel. Why? Think of it from his perspective.  If Ramiel and the other Princes of Hell (PoH) turn against Crowley, he'll need potent allies and Winchester & Co. have proven themselves there. However if Ramiel dies, it is more likely that the PoH will turn against Crowley and potentially destabilize his position. So obviously he wants that guy mollified and safe as well. Hence at first he tries to separate and broker a peace between both sides because his position is most secure back in the status quo.  Once Ramiel confirms that the PoH will not be allies of Crowley, he then throws his full lot in with the Winchesters.  Now the question will be can one of the PoH take back the kingship of Hell by command as easily as they gave it or will Crowley be able to fight them and hang onto it?
Still we have the question of why does Crowley help Castiel at the end? We have two obvious reasons: 1) The spear is a threat to him.  It killed a PoH, it can kill the King just as easily.  By saving Castiel, Crowley really helps himself by removing a dangerous tool from the boys' hands, yet maintains a cover of being an ally. 2) Like we said, if the PoH turn against him, Crowley will need help and Castiel owes him a favor now. All of this follows logically from the story presented and the characters' actions which is far better than Crowley has had in the last few seasons where often his actions were of the "because he's a main character" reasoning.
Speaking of, the basic prop was delivered to the audience textbook perfectly. See, the problem with fantasy & science fiction is that the storyteller must often TELL the audience what is going on in addition to SHOWING what is hapening. This has been the big flaw of the previous Carver years. This episode had a scene which shows us the weapon, AND then it proceeds to tell us about it and what it does. Namely, it kills demons instantly and slowly kills angels. Why does it do that? Because of these runes on it and because of Michael's motivations as a character. At least I hope the runes weres a part of the workings instead of the sum total because otherwise the implication is that the boys could scribble down those markings and begin killing demons with paper cuts - which would suck for tension. At least it is left open the question of whether the spear can be rebuilt. Anyway, later on we have a real dilemma in the story. Our heroes have this great new weapon that can kill some of their worst enemies, but Castiel is dying thanks to it. Destroy the weapon - Castiel is restored. It makes perfect sense in the logic of the story and world and characters. And we the audience understand and grasp this dilemma all thanks to fifteen seconds of exposition.
Now let's go back and look more at this retcon of 6 years ago. What else are we shown? A high ranking demon that doesn't care about the politics of the spirit world, he just wants to chill out on earth. How did Azazel put it waaaaay back in "Lucifer Rising" (S4e22)?
I have been searching for you for so long. You have no idea. The others have lost faith. Dickless heathens. But not me. -Azazel
At the time of air, the implication was that Azazel alone was faithful of all the demons in Hell (given lines from earlier demons and Ruby's later revelation). But now with this new retcon, we can ask ourselves: what if he wasn't? When we look at that line, what if he's talking about the other PoHs? Ramiel's line about wanting to be left alone is in keeping with the previously established lore, adding new dimensions to Azazel's frustration and explaining where these characters have been all this time.
Now when did these princes get out? Were they released along with Lilith when the gate was open in "All Hell Breaks Loose p2" (S2e22)? Probably. If so, then this retcon adds new explanations to the problems the demons had in S3. Now while rewatching with the knowledge that Azazel is dead and the other PoHs are more interested in chilling out than fighting, we can understand now why some demons may adopt a "let's chill out" stance (like in "Sin City" (S3e04)) while others wanted to rally around Lilith and "raise up Lucifer." This is especially great since the dropped thread of a demon civil war was one of the confusing parts of Kripke's era - what did the demons disagree about? Now we have plausible answers.
Again, THIS is an example of a good retcon, it goes back and ANSWERS more questions about the past than it raises. It brings new light and understanding to past events. Unlike the earlier attempts at establishing a "new, strong, evil power", the motivations of the PoHs established here explains why these guys were never around during the Leviathan or even Abaddon's shenanigans. It's beautifully elegant in its simplicity: "they didn't want to be involved." Were they around during Lucifer's rampage in S5? The episode doesn't say for sure meaning that we could say yes and explain further why they want to stay out of the world's way. We could say no and explain just how much work they put into staying underground. Do you see how the retcon answers the most important and vital questions to making the story work immediately but allows other questions to remain open and be answered later? This is how you do a retcon.
Especially compare this episode to the one introducing Cain. In both we find an isolated, bearded demon going about his business when Crowley and Winchester(s) show up to wreck his day. However Cain's motivation was a bit nonsensical, like why did Dean have to "prove himself?" What did he prove to Cain? It's not really explained; we the audience have to invent a reason.  By contrast, Ramiel's motivation was kept simple and makes sense. We could understand what he was about and why he took the actions he did. Let's be honest: Cain only worked because Timothy Omundson is such a talented and skilled actor.  Yet in the logic of the show and canon, he really didn't. (Like why was it until 1863 that he reformed? If Cain is literally true, how is Prometheus literally true?) Heck now he even makes less sense since by a line in this episode about Lucifer making the Princes, Cain should technically be one, but he's a Knight of Hell? Nevermind best to keep pushing S9 further and further out of the canon...
In this episode we also had confirmation that Ruby's knife definitely won't kill a PoH. Therefore it seems unlikely it was ever going to be able to kill Lilith even though the boys thought it could.  This brings up an interesting possibility that Ruby might have sabotaged both her knife and the Colt to keep it from killing the higher ranking demons (and Lucifer).  This makes sense since she would be a loyalist to Azazel and Lilith and, in that light, her end plan was to force Sam to kill Lilith using his mind powers, thus breaking the seal. If he could have used the Colt or Knife to kill her, that plan would have been ruined, so we have another retcon possbiility, which would iron out the roughest patch in Kripke's tenure. In fact, SPN writers, if you guys want to use that retcon to set up some stories, PLEASE do so. It's all yours, free of charge.
Of course a good story isn't everything about a TV show (just 80%).  This episode also worked because the cast and crew took what would have been a pretty dull plot that was nearly a rehash of the aforementioned Cain episode, and spiced it up by filming and structuring it in a tribute to Quentin Tarantino's Reservoir Dogs and Pulp Fiction. (I'm not much of a Tarantino fan, but I did recognize it.)  It's important for shows to always strive to tell their stories in visual innovations to keep things fresh and interesting.  This is part of what made "Baby" (S11e) the stand out episode it was.
Oh yeah, and the best reason why the episode worked and was of such good quality?
 2nd best girl