It was a normal Sunday afternoon in the Ponyville library, the day after Trixie upended the town with the Alicorn Amulet. As Spike was busy erasing Twilight's annotations on a few of the public books, he realized something odd about the previous day's events.
"You know what's odd, Twilight?" Spike asked.
"What?"
"When we disguised Fluttershy as Rainbow Dash during your magic duel with Trixie, and you used a spell to change her eye color, ponies couldn't tell the difference between them. I mean, that's odd isn't it? Their facial features and body shape are exactly the saaaaame."
"Oh, that," Twilight said. "Everypony has that property. It's our manes, cutie marks, and personality that make the difference."
"Whaa-?" Spike said. "That's so cool. We dragons are totally different. You mean to tell me that your coat color, mane, and cutie mark are the only things that differentiate you from any other mare?"
"Well not exactly," Twilight said. "I don't look like Mrs. Cake, for example, but for the large part that's right. It's Clover's Second Property of Isoponificatio-"
"This is awesome! Twilight! That means all of you can turn into Pinkie Pie and pretend to be the second coming of the Pinkie Pie clones. That means anypony can turn into Trixie and star in a Trixie vs Trixie magic duel. That means ... ooh, you could turn into... you could turn into..."
"Turn into what, Spike?" Twilight asked.
"That means you can turn into Rarity," Spike said, his eyes all cute. "Then we can reenact .... ze loves."
Twilight Sparkle sighed. "Seriously Spike? I don't think spending an evening pretending to be somepony I'm not is a good idea, especially when the real pony you want lives in this very town. I mean, if you want to date Rarity, just talk to her. It's not hard."
"She wouldn't have me!" Spike said. "I could ruin one true chance of love by acting too soon! When my fire breath comes in and my spikes get bigger, maybe she could have me then. But alas, Rarity only sees me as a little brother!"
"Makes sense, I guess," Twilight said. "I get that love can be hard sometimes. But I'm still not coloring myself as Rarity for you."
"Or what if..." Spike said. "What if.... what if none of us are really the ponies we think we are? What if our friends got taken away and replaced by some other pony who changed their colors? What if you were secretly Rarity all along?"
Twilight chuckled. "Spike, if any of our friends weren't who they were to begin with, we would have noticed. We each have our own unique personalities, our own lives and loves and things we're passionate about. It's not just about appearance. Surely you like Rarity for her personality traits as well?"
"Of course," Spike said, putting his fingers together. "But... I won't pretend she isn't beautiful, a mare like her. Besides, I need to practice my moves for dating her, and I don't want my first run-through to be with her on the first date! Let me practice with you, Twilight. Come on, help your most loyal assistant."
"Spike," Twilight rolled her eyes. "This is silly. This whole business is too changeling-y for me. I only did the thing with Fluttershy and Rainbow Dash because I needed to save Equestria. Rarity's fond of you. Just go and ask her out. I'm not going to change myself into Rarity to help you dither on your affections."
"Oh," Spike said, suddenly devious. "You know, you talk a lot about going and asking her out. But if so, why haven't you ta talked to Soarin' yet? It's obvious you've got a crush on ....."
Twilight Sparkle's face turned red.
"What?" she said. "Wh-what are you talking about?"
She leaned in and whispered, "What gave it away?"
Spike smiled. The balance of power was shifting back in his favor. "You know, if you just help me out a little here, no other pony would have to know."
Twilight Sparkle relented. "Okay, fine. I see that your heart's in the right place. Here's the deal: I be Rarity for two hours, and you can practice your charm on me. I'll even do a voice spell to capture her voice perfectly. Just don't do anything crazy, okay?"
"Awesome!" Spike said.
"In return," Twilight went on, "you have to do extra hours at the library and make sure all the new books on quantum chemistry that Derpy returned last week are entirely in order, cataloged, and put in their proper place by tomorrow. All fifty of them - that mailmare sure is a voracious reader. Do we have a deal?"
"Yeah, of course..." Spike said. His eyes went all dreamy-like.
"Good," Twilight said. "I'm going to get ready. Why don't you go send a message to Fluttershy in the meantime, and tell her that the bunny feeding at 2 PM tomorrow is a go?"
"Sure," Spike said, leaving to go. Twilight Sparkle went down to her library.
"Spells, spells, .... ah. This one should do it."
******
When Spike came back, the second most beautiful pony Spike had ever seen was there to greet him. Twilight had such a perfect rendition of Rarity that he could hardly tell the difference. As usual, Twilight's attention to detail showed; she even got birthmark on the small of Rarity's back was present, and her mane was done just right. Still, it wasn't perfect: Twilight never could match Rarity's subtle hip sway.
"Wow Twilight, you did it!" Spike said. He jumped up to give her a hug. "Now remember Twilight, try and behave like Rarity would. We should play some gemstone ball. It won't be easy to get time with the real Rarity like this - or to try and practice my smooth dating skills with her. Now, where were we? Ah. I have this plate of gemstones I would like to show you, Miss Rarity..."
*****
Twilight Sparkle, locked in her laboratory, was reading a book on young dragon psychology by the light of a candle.
"Whew," she said. "Close one. I'll never understand how dragons think."
Ten minutes ago, she had teleported into Carousel Boutique, surprising the resident fashionista with her sudden appearance.
"Rarity, I need to ask you a favor," Twilight had said. "It'll be just a few minutes before Spike gets back to the library, and ..." she leaned in and whispered.
"Sure, spend some time with dear Spikey Wikey?" Rarity said. "That does sound like a fabulous way to spend an afternoon. But - you'll have to come to spa day this coming weekend. Full treatment."
"Deal," Twilight Sparkle said. She could always get Pinkie Pie to wear a fake horn and change her mane colors - but she had a feeling Pinkie Pie's cover wouldn't last *quite* as long.