(Relevant self-pimpage: I'll be doing a panel-presentation-thingy called "Deconstructing Equestria" at Everfree Northwest in Seattle this Friday morning. Come say hi!)
Season 2, Episode 26: "A Canterlot Wedding, Part 2"
As Twilight Sparkle and Real Cadance try to escape the caverns, Evil Cadance prepares for the wedding, both Cadances duet in the episode's showstopping number, "This Day." I don't know the exact term for this kind of song in the musical theater context, the second-act finale where the characters all sing a song together while being in different places, but for me it brings to mind "La Resistance" from South Park: Bigger, Longer, and Uncut, and "Walk Through the Fire" from the Buffy the Vampire Slayer musical episode. In good ways, obviously.(As an aside, I can't help but wonder how flower girls Apple Bloom, Sweetie Belle, and Scootaloo would feel if they got their cutie marks for that. It would have to be a bit of a letdown.)
The marriage of Evil Cadance and a hypnotized, green-glass-eyed Shining Armor is about to be completed by the unobservant Princess Celestia ...... when Twilight Sparkle makes yet another melodramatic entrance, this time accompanied by Real, Not-Evil Cadance. After the rest of the Mane Six Plus Spike get over the momentary (but still heartbreaking) disappointment in Twilight and realize that something weird is going on, it's up to Applejack to ask the pertinent question of "Wait, what?" Real Cadance explains that Evil Cadance is a changeling, a shapeshifter that feeds off others' love. And since Real Cadance had to go and say that, Evil Cadance reverts in a burst of freaky green fire back to her true changeling self. The Changeling Queen explains that the love-heavy Equestria is basically an endless smorgasbord for changelings, but Cadance is sure that Shining Armor's force field will protect them. The Queen doubts that, and -- oh, hi, Derpy! Final Derpywatch of the season, six o'clock. Unconsummated marriage or not, the Queen says Shining Armor is under her total control, though not quite total enough to get him to stop making the force field. But it's getting weaker, and the changeling hordes are slamming away at it. Well, that settles that, then. The Changelings will take over Canterlot, then all of Equestria, and then -- Hi, Princess Celestia! You kinda disappeared when the going got rough. Got called away to another one of those emergency meetings with the Ponyville Mayor, I'm guessing? Or maybe you stepped out to guard the day? Anyway, good to have you back. Celestia says that even though the Queen has prevented Shining Armor from making the force field (give or take the fact that the force field is clearly still there), she is still determined to protect her subjects. Celestia and the Queen duke it out, Egg Shen vs. Lo Pan style ... ... but Celestia gets knocked out of her crown and flat on the ground, her horn burnt, another violent, unsettling image in an episode full of them. But this isn't the first time Celestia has been unable to defeat an enemy, hence the Mane Six being called into service in "Friendship Is Magic, Parts 1 & 2" and "The Return of Harmony, Part 1 & 2." And like those prior two-parters, Celestia charges them with retrieving the Elements of Harmony, her solution to everything. No problem! They start making their way to (I'm guessing) Canterlot Tower to get the Elements, but unfortunately, the changelings have finally managed to destroy the force field that Shining Armor hasn't been able to make for a while now. (And as a matter of fact, I am an expert on unicorn-generated force fields.) As though it weren't bad enough that there's now a swarm of changelings in Canterlot, they're changelings that can change into other things, as is their wont. Being changelings and all. Among the other things they can turn into include Rainbow Dash ... ...and Pinkie Pie and Rarity and Applejack and Fluttershy and ... Fluttershy? Oh, you did not. Rumble! The smoke clears, and ... the non-changeling Mane Six has won the battle! And Fluttershy hated every second of it. But our heroes are nowhere close to winning the war, since there are plenty more changelings where those came from, and they got to Canterlot Tower first. The Mane Six are taken to the Chapel, where Shining Armor is still a green-eyed zombie, Cadance is stuck to the floor by a freaky green goo, and a still-alive Celestia is imprisoned in a freaky green cocoon-y thing (presumably filled with said freaky green goo, ick). Since Twilight Sparkle is too classy to say "I told you so!" re: the whole Evil Cadance thing, the Queen says it on her behalf. In a brief reprise of "This Day," the Queen gloats as she looks down at the now war-torn Canterlot. As the Queen is distracted, Twilight frees Cadance from the goo, and Cadance uses her love-horn to free Shining Armor from the Queen's spell. See? I told you it would pay off! Granted, as a story point would work just as well if Cadance wasn't able to use it on complete strangers, if it was purely love for Shining Armor that set him free and not a pre-established superpower ... but it still gets me gooey inside every time, as does the combined power of their love defeating the changelings. It's reminiscent of the climaxes of the previous epic two-parters, what with the levitation and glowiness and a shockwave of magic vanquishing the villain (though the changelings aren't "vanquished" so much as "scattered across Equestria"), but this is the first time the big glurge feels organic to the story, and the music is just breathtaking. It's also worth noting that neither the Elements of Harmony nor the Battle of Canterlot had anything to do with the victory. The Battle was a thrilling scene and I adored it (Pinkie Pie using Twilight Sparkle as a gun notwithstanding), but ultimately it was nonviolence and love that saved the day.