This is where Chrysalis really manages to shoot herself in the hoof (those holes had to get there somehow). Not only does she confirm to Twilight that she is indeed evil, but she also imprisons her right next door to her other prisoner, and in fact seems to specifically engineer a meeting between them, allowing them to team up. Maybe she was hoping Twilight would be angry enough to just kill the real Cadence outright, especially as she seemed intent on taunting her to raise her ire, but it seems a poor gamble to take when you’re on the cusp of victory. It's going to backfire horribly, as we’ll see.
A quick song-and-dance greeting ritual establishes the true Cadence’s bona fides (a little too quickly, if you ask me) and they team up to escape. This also makes the “hope for friendly fire kill plan” even more unlikely. Chrysalis knows it failed because she sends the bridesmaids to block their exit, but still laughs as if everything is going according to plan as she leaves, which is odd, to say the least.
Of course, as stupid and self-defeating as Chrysalis’ action is, it also results in the best song of the episode, "This Day Aria." What else is there to say, it’s a perfect song. It also moves the action along quite nicely, as Twilight and Cadence make their way through the cavern while the wedding continues above them. No pony notices the groom’s zombified expression and glowing green mind control eyes, for some reason.
After Twilight spots an exit, she teleports herself and Cadence to it, only to have the bridesmaids step in to stop them. Yet more questions: what did Chrysalis originally mind control them for, anyway? They vanished before Twilight ever confronted her, so they can’t have been waiting down there in order to block the way the whole time. Unless Chrysalis expected Cadence to make a break for it herself, in which case, why did the latter wait for Twilight to find her before trying to get away? None of this makes much sense to me. The way the pair escapes is also a bit of a head-tilter. The bridesmaids run off after the bouquet that Cadence quite literally makes appear from nowhere, in a bit of totally unnecessary fridge logic. (Welcome to Plot Convenience Theatre!) That would have been a great time for Twilight to use the Want-It-Need-It spell from “Lesson Zero”; they could literally have used a rock for the same purpose without creating yet another flaw for me to scratch my head over, and added a lovely little continuity nod at the same time.
When the pair makes it back to the wedding, it sure is nice of Cadence to wait for Twilight to burst in first, just so everypony can look unhappy to see her. Confronted by the real bride, and having her secret being revealed, Chrysalis returns to her true form, ignoring the fact that freaking Princess Celestia is officiating the wedding. As it turns out, though, Chrysalis is now strong enough to win the Beam Spam battle, slapping down Celestia like an unruly child. (Well, not really, even she was surprised she won, but just go with it) Now this I like, as it destroys the impression that the alicorn Princesses are somehow living gods, which I never approved of anyway. The (now second) most powerful magic wielder out of commission, the time has come to fall back on the last line of defense for Equestria, the Elements of Harmony. Which are locked in a tower. Which is protected by a spell only Celestia knows how to bypass. Hmm. It seems like Chrysalis isn’t the only poor planner in this episode, after all.
All of which is moot, since despite a great action fight scene, the Mane 6 are captured before they can get to the Elements. All seems lost… the shield has failed, Celestia is now a pod pony, the Mane 6, Cadence and Shining Armor are prisoners, and the changelings are roaming around in their true forms, feeding from the fearful ponies of Canterlot. Wait, what? If changelings get power from love, how does fear sustain them? If they can suck the love out of somepony in their true form, and while the pony in question is fearing or hating them, why do they need the ability to take on the form of a loved one in the first place? I’ve been scratching my head so much, I’m right through my skull and hitting pink stuff directly. Quick, Chrysalis, sing an ironic reprise of an earlier song to distract me!
Once again creating the impression that those holes go through her brain as well as her legs, Chrysalis not only leaves the Mane 6 free, but allows Twilight to free Cadence, who in turn breaks Chrysalis' hold over Shining Armor. I mean, being sure of your victory is one thing, but allowing your prisoners to do everything in their power to stop you when you shouldn't really be giving them the chance is simply stupid. Again. Such arrogance goes well beyond just being a fatal flaw, it's downright perverse. Unfortunately, par for the course for MLP villains.
Cadence's love gives Shining Armor the power to cast his protection spell. A lovely little animation error gives Cadence her crown back before she even begins, but I like the idea that the magic repairs all her scratches and other evidence of her underground incarceration. Sort of a "beauty is never tarnished" moment. It also briefly gives both her and Shining Armor's manes the wavy look of the alicorns, although if this is a true change or just their manes blowing in the breeze, I'm not certain. Here's another bugbear of mine. People claim that the spell is the power of love used as a weapon, in much the same way as the Elements of Harmony. It's not really, it's just Shining Armor's protection spell. Cadence is only using her magical lurrve powers to give him the strength to cast it. Anyway, the spell manages to knock all the changelings away (again, is that spell specifically targeting changelings?) sending them flying back to the place they came from. It also seems to dissolve the changeling's mucousy stuff, while not touching any of the ponies.
Action over, it's back to the wedding, with real Chrysalis just loving everything about the planned ceremony. I guess that's understandable. It's got to be better than what she started the day expecting to face. So ceremony, kiss, Sonic Rainboom on command. Then a slow dance, Luna pitches up after being who knows where, ponies sing ("Love is in Bloom" is pretty standard, a nice tune but nowhere near "This Day Aria") and dance and cry and laugh and all that good stuff that a wedding brings. And Spike brings out his bachelor party brick joke for an everypony laughs ending. Oh, and fireworks.
Pros - The songs are pretty good. Despite it's flaws, the story is entertaining. The scene where they fight the Changeling horde is wonderful, and harkens back to the days of the Powerpuff Girls.
Cons - Shining Armor and Cadence (a new alicorn) are thrown in, bam, no warning from any of the other 50 episodes in the series. The villain apparently goes to great lengths to ruin her own plans. A lot of stuff doesn't make sense.
Overall, despite it's flaws, the story is really enjoyable, and has some nice touches. If anyone can provide some possible justifications for the flawed logic, I'd be happy to hear what you have to say. So, now that we have had our happy ending, all that remains to say is this: Until next time, keep on chasing those rainbows!