"Ah, good morning, Rainbow Dash." said Rarity, trotting along.
"Ah-heh. Hi, Rarity. Um, aren't you supposed to be watching your shop or something?" asked Rainbow, nervously.
"Oh, no, it's closed today." said Rarity, waving a hoof. "Today's my spa day, I was just on my way to Fluttershy's cottage. And who is your, um colourful friend?"
"Ah. Rarity, this is Spectrum Slash. Spectrum Slash, this is Rarity. Annnnd now that we're all introduced, the two of us really should be moving on, and -"
"You're White." said Spectrum Slash, bluntly.
"Um... yes?" said Rarity.
"You're this universe's Fashion White." Spectrum Slash stepped rapidly forward, pushing her face into Rarity's. "Are you evil?"
"Me?" asked Rarity, leaning back. "No!"
"Hmmmmmmmmmmm." said Spectum Slash, sceptically. "Have you ever put a lame and frilly dress on somepony from ambush?"
"Back off!" said Rainbow, grabbing Spectrum with one hoof and pulling her back. This left Spectrum's rainbow trail in Rarity's face instead of Spectrum herself, which was only a slight improvement, but it was an improvement nonetheless. "She said she's not evil, right? So she's a good mare. She might look like Fashion White, but she's not the same pony, okay?"
"And, um, just who is this Fashion White, exactly?" asked Rarity, taking another small step back.
"Ah-heh. Now, that's a lon-" began Dash, before Spectrum interrupted her.
"My first, and worst, antagonist." said Spectrum, stalking her way around Rarity like a cat circling prey. "A white unicorn. Like you. With a purple mane. Like you. And an evil, evil, evil mind." Spectrum completed the circle around Rarity, surrounding her with a barrier of rainbow, and continued circling. "She would skulk around Awesomeville. Hiding from the ponies around her. Then jump out at them, using her magic to force them into lame and frilly dresses that reduced their awesomeness by at least twen-"
A sudden burst of rain landed on Spectrum's head, soaking her coat and washing the rainbow out of the air around her. Rarity looked up, and saw Rainbow Dash standing atop a depleted raincloud.
"My goodness," said Rainbow, "you appear to have been rained on! Come on -" Rainbow zipped down below the cloud, and began tugging Spectrum away - "I'm sure we can find a towel for you at Applejack's place, before you catch a cold! Bye, Rarity!"
"I am too awesome to catch colds." objected Spectrum Slash, as Rainbow dragged her away.
Rarity watched them go for a while, then sighed. "I wonder what that was all about." she murmured, before stepping out of the rainbow ring through the gap Dash had left and trotting towards Fluttershy's cottage again.
~~~~~
Clouds, thick, dark, black gathered over the land. A wind blew, a wind that promised rain, and lots of it. It howled through the streets of Awesomeville, where ponies ducked and hid, waiting for Spectrum Slash to arrive and save them.
In the town square, every pegasus gathered together. At a given signal from Yellowshy, they all acted in perfect synchronicity; each pegasus folded their wings tightly to their sides, fell over, and froze up. In chorus, every single pegasus said "I am scared!" at the top of their voice.
And then they waited.
~~~~~
In Canterlot, at the top of the castle's tallest tower, Princess Celestia confronted the cause of the cloudy chaos.
"You shouldn't be doing this, Good Discord." she said. "I know your heart's in the right place, but Spectrum Slash handles the weather."
"It hasn't rained in a month." said Discord, grumpily. "Your rivers are already almost dried out. Surely a little bit of moisture would be good?"
"No!" said Celestia. "Spectrum Slash's weather schedules are hard to understand, but there's a reason for it! Any extra rain now could be disastrous!"
Discord glared at her for a long moment, then twitched a single talon.
"What?" asked Celestia. "What did you do, Good Discord?"
"One drop." said Discord. "In Appaloosa."
"Oh, no!" Princess Celestia ran downstairs. "Quickly, everypony!" she called out into the echoing halls. "Mobilise the guard! Go evacuate Appaloosa!"
"What, you're evacuating over one raindrop?" asked Discord. "In the desert?"
Another thought occurred to him. "And... you're not leaving this to Spectrum Slash?"
"We've got to save ponies while Spectrum deals with the flood!" insisted Celestia.
"...wait, flood?" asked Discord. "I let one drop fall!"
"Go to Appaloosa, then!" yelled Celestia. "See what your 'one drop' of unauthorised rain has done!"
Discord shrugged, and snapped his talon.
~~~~~
By the time Discord arrived in Appaloosa, it was shoulder-deep in water. That is, it was up to Discord's shoulders.
"This is RIDICULOUS!" he yelled. "It was one drop!"
"Glub glub glub glub glub." opined a passing pony, drifting on the current.
Discord sighed, and snapped his talon. The flood shrank down, to a bluish-coloured marble, which he tossed up and down in his eagle claw.
"....one drop." he said, finally.