Overture
"We came as fast as we could. What's the case?"
Canterlot City laid quiet as always. Only when coming close would one be able to hear the gentle buzzing of those basking in the glory of another warm Wednesday afternoon, as well as three multicoloured specks that had just stormed into the castle's throne room. The large white speck in the same room moved from its seat and said, "Ah, I'm glad you could respond, my faithful student. Equestria lies in the path of a great peril, one which you may already see coming on the northern horizon."
"Wait, the Maresoon? Doesn't that come around like once a year?" Rainbow Dash stepped forward. She only remembered proper protocol when a unicorn's hoof from behind forced her to bend. "Hey, not in front of them, Sparky."
"That it does, but for some unknown reason, it has become much stronger." Celestia's horn lit up for a second as she conjured a spectral image of a whirling cloud between them. "We need able ponies to investigate and battle the cause."
"You can count on us, Princess!" exclaimed Twilight, stepping forward.
"Actually, Twilight, I was thinking more of Rainbow Dash and Fluttershy as they have something you simply don't," said Celestia in a well-chosen, soothing tone as if not to agitate Twilight. "You'll still be vital to the mission, of course," she continued, restoring the spark in Twilight's eyes. "They'll need somepony to monitor their progress and report it to me. On that note, did you not bring Fluttershy along?"
"She's here, princess." Rainbow stepped aside faster than anticipated, revealing a huddled pegasus hiding behind her voluminous mane. "C'mon, 'Shy," she said, trying to make her stand up. "Equestria depends on you! You can't just run away from everything!"
"It worked until now, so yes, I can!"
While the two pegasi were busy arguing with one another, Celestia adressed Twilight. "I trust that she'll be coming along. Are their affairs in order? I don't expect them to die, of course, but it will demand that they leave Ponyville for a bit of time."
"They are. Made it a habit to order them whenever Sparky comes up with another Equestria-threatening crisis. You'd be surprised how often she does that." Dash turned her attention back to Fluttershy. "And so are yours. C'mon, we'll tell Applejack, she can take care of all your critters while you're gone. Aren't you officially a fog specialist? It's just a huge lump of fog, piece of cake. You're coming along whether you like it or not, you know that, right?"
At long last, Fluttershy gave up resistance. "Alright, if you really want me to come... but why me? Why not somepony more qualified, like Cloud Chaser, or Speeding Bullet?"
Princess Celestia walked up to them. "Let us say that you two share a bond which the others do not. Thank you for your cooperation, Fluttershy, your help is vital to this operation. Now, if you all would follow me to the supplier's." She magicked open the wide doors and led the way through the castle.
A few minutes later, four ponies arrived in the castle armory's side room. It was a curious little chamber about which few ponies knew, filled to the brim with whistles and whizzers, odds and ends, and gadgets and gizmos. From between all the clutter, a white unicorn stallion with a cogwheel cutie mark hurried towards them. "Ah, princess, welcome back into my little domain." His eye shifted from Celestia to the others. "These are your volunteers, I suppose? Where are my manners. Good day to you, miss Sparkle, and to you two, my name is Cue Applause. No, seriously." He extended a hoof towards them. Rainbow took it, more out of automated manners than anything else. "For short, my friends call me Cue. Don't worry, you can too. Now, to the equipment that you'll need. Packs, bottles, rations, all the boring stuff." He led them forward to the room's main table. "And here comes the interesting stuff." From the chaotic clutter, he pulled forth a small box emitting periodic beeps. "This is an antiproton detector. We don't really know what they are yet, but if there are any where you're going, this will find them." He tossed it into a brown bag standing on the table and instead took from the mess a long metal horseshoe. "Hoofblades."
"Little more than clunky in the wrong hooves, a deadly weapon in the right ones," Rainbow commented. Upon catching Twilight's eye, she added, "There are a lot of things in my past that I don't really wanna explain, but those things are awesome if you've got the mobility to use them."
"Ah, wonderful. Now, for reporting, you can use this seeing as neither of you is a unicorn." A large violet candle floated out and into the bag. "Imbued with dragon essence to gain the spell they are most known for. Don't ask how we made this. If you burn any letters in its flame, miss Sparkle should receive them. And for close communication, these two are linked, so you will be able to exchange notes. State of the art magic." Another two smaller candles also made their way inside the growing pack.
"Tsk, foalsplay," murmured Twilight.
"Think you can do better, Twilight?" he responded.
She levitated two pieces of wire, wrapped them in cloth, and shut her eyes. For a moment, both the aura around her horn and that surrounding the wires lit up brightly as the setting sun, before it perished and they set themselves into the bag. "There. Now they'll transfer between themselves anything that you're saying, without the hassle of writing. Don't thank me, all in the service of Equestria. And proving old Cue wrong."
"Isn't she a beauty. I believe that should about do it. Which of you is the stronger - ah, the famous Rainbow Dash, of course." The bag from the table floated up into the air where it met up with the supply bag and a harness which, with all the provisions attached, fastened itself around Rainbow's sides. "Best wishes, both of you."
"The Maresoon is currently near the Crystal Empire, Princess Cadance reported its unnaturality." Princess Celestia pointed towards the train standing ready in front of them. "This train will take you close enough that you can reach it by wing. You should also be able to return there for supplies if needed. I've had ponies reshape an outer, slower part of it for use as an outpost. Your mission is to investigate and disable the cause. I will expect a report daily or whenever you find something significant." She bent her head slightly. "Good luck, my ponies. Everyone counts on the two of you."
"We'll do as we must, princess." Dash did an overdrawn bow, then turned to Twilight. "I'll be back soon. Don't go getting yourself killed." She claimed one last, extra-long kiss, then headed inside the train car.
After many formalities and reassurance, Fluttershy also hopped on board and the train set off. All too soon, it disappeared within Canterlot's wide range of tunnels, leaving only one's imagination to show it heading off to the darkness on the horizon.