Twilight Sparkle scribbled on her chalkboard, moving through the equations lightning-quick. Though she was at a bit of a disadvantage when it came to writing, she was on the verge of a breakthrough, and Twilight wasn’t about to let something like a lack of magic stand between her and the greatest scientific discovery of all time.

She had shut her friends out, but they understood, at least at first. Twilight had been laughed out of every university she had ever spoken at when she presented her theory. They understood that Twilight needed this if she was ever going to be given the respect she deserved. This was more than just a career for her now, more than just an fixation on the unknown. This was an obsession.

“You should take a break, Doc.” Twilight barely processed her assistant’s voice, too busy focusing on the formulas and equations before her. Spike was far from offended by this. He had long been used to Twilight locking herself away, starving and dehydrating herself, until she made some sort of a breakthrough. So it was with measured practice that he cleared off a section of Twilight’s desk and sat down his tray of tea and sandwiches.

“Something’s missing…” muttered Twilight, running a hoof through her mane. “Something’s not quite right…”

“Doc. Please, eat something,” begged Spike. “You’ve been cooped up in here for days now.” Still, Twilight ignored him. “Doctor. Doctor!”

“I’m not a Doctor!” snapped Twilight. “Not anymore! These… These morons! So close-minded! Why can’t they see what I see? It’s so obvious!”

“Of course. I understand, I—”

“Do you understand? Because I really doubt that. How could a small, mindless little dragon possibly understand this?!” Twilight stomped her hooves angrily. “Nopony understands, because they’re all idiots! Brainless, dim, slower than apes!”

Spike watched his friend as she processed the words she had said. Her expression morphed from rage to the most solemn of regret, which let Spike know that he had no reason to be angry with her. She already hated herself more than he could ever make her.

“Spike… I’m sorry. So, so sorry,” said Twilight. “It’s just… The math, the science, it’s all very frustrating. Sometimes, I feel as if the numbers are changing, as if the very fabric of the universe is shifting to evade me.” Twilight shook her head firmly. “But that doesn’t excuse me for my behavior. You deserve better than that. So I sincerely apologize. How can I make it up to you?”

“You really wanna know?” asked Spike. Twilight nodded. “Come eat. Take a break. Go visit your friends. They miss you.”

“Spike… You know I can’t face them. Not after what I said,” sighed Twilight. “Rarity was so angry, she refused to clear the rain around the lab for a month.”

“But she’s over it now. She flew back home to Cloudsdale for a bit and thought things over,” said Spike, pouring Twilight a cup of tea just the way she liked it; Two sugars and a dash of cream. Twilight nodded her appreciation before tearing herself away from her board and taking a seat beside Spike.

“I was so horrible to them,” said Twilight. “I was rude and selfish and disrespectful, and for what? I’ve got nothing to show for it. Nothing but years of regret and wasted time.”

“They’ll forgive you, Twilight,” said Spike. “One trip to Pinkie’s farm, and things will be back to normal.”

“I don’t deserve their forgiveness…”

“No. You don’t. But that doesn’t matter. They’re your friends, and they’ll forgive you whether you like it or not.” Spike passed the tea to Twilight, who took a calming sip. It reminded her of a better time. Before she had become so obsessed with what she called “The Other Frequency”. Before she had given up on friends and love and family. Before she had lost it all.

“Do you really think so?” asked Twilight.

“For sure.”

Twilight sighed, then grabbed a sandwich from the platter. She had spent her entire life studying the universe, and what may lie beyond it. Years and years of sharpening her mind, learning the mechanics of her reality, understanding everything just to discover something new that invalidates all she knew before, and it was all for nothing. There was no question in Twilight’s mind that she was the greatest earth pony mind in Equestria. Now, for the first time in her life, she doubted how much that mattered.

“Well, Spike,” said Twilight with a sigh. “Maybe it’s about time that I took a sabbatical.”

“That’s good, Twilight,” said Spike with a grin. “Come on, let’s have lunch and go see the girls.”

There was an earsplitting shriek, like the squeaking of rending steel.The air in front of Twilight’s chalkboard began to shimmer and shake, then split into a void of swirling lavender light. Spike and Twilight backed away, mesmerized by the swirling smoke that emanated from it.

“What in Luna’s name…?” Twilight stared, utterly perplexed. “Spike, get back.”

“What’s going on?” asked Spike.

“There’s no time for that! Get behind me!”

A hoof poked through the void, bringing with it a wave of heat and a gust of air. A unicorn stepped through, wearing an all-black bodysuit and a helmet that obscured their face. It wore a belt around its waist, strapped with several tools and devices the likes of which Twilight had never seen before. It pressed a band around its hoof, and the portal behind it quaked and shook before collapsing onto itself.

“W-w-who are you?” asked Twilight. “Why are you here?”

“Hm. Earth pony,” said the unicorn. Though it was muffled by the helmet, the unicorn’s voice was distinctly female. “Interesting. I didn’t realize there were any of you left.”

“Where did you come from?” asked Twilight. “Who are you?”

“To avoid confusion, you can call me S-169,” said the unicorn. “But it doesn’t much matter.” She pulled her helmet from her head, setting it down next to her. Twilight’s eyes went wide, then a smile spread across her face.

“I was right… Sweet Luna, I was right!” squealed Twilight. “Spike, I was right! I was right!”

“Ugh. Theorist,” sighed S-169 with a shake of the head. “Get a grip of yourself, Sparkle.”

“This is way weird,” said Spike. “You’re… But you can’t be.”

“You’ve kept your Spike. That’s good,” said S-169. “The last three I visited had split up, so I had to chase them down.”

Twilight laughed giddily, running over to her desk and grabbing her papers. She flipped through her calculations. This was what she needed all these years. S-169 could be the breakthrough she needed to get her Ph.D. reinstated, to have her life’s work validated, to make all these years of strife and solitude finally worth it.

“How did you get here? Can you help me work it out?” asked Twilight. “I’ve been struggling to crack it for the longest and there’s something that just won’t fit!”

“Um… Twilight?” said Spike nervously.

Twilight turned, her grin faltering at what she saw. S-169 held a small black orb, about the size of an orange, which she gently rolled forward. S-169 returned her helmet to her head and tapped on the band around her hoof, reopening the portal she had come from.

“Yeah, the charge is armed. I’m headed home now,” said S-169, touching a hoof to her helmet. “This is Dimension R-322, by the way. Mh-hm. Okay. Heading there now. Hang in there.”

“W-wait, what?” Twilight stepped forward cautiously, tapping at the orb S-169 had tossed. It began to glow dark red, letting out a high-pitched shriek.

“Sorry, R-322,” said S-169.

“What in the world?” Spike approached the orb himself, lifting it up. “What is this thing?”

There was a series of beeps, and then a flash of bright blue light. Spike and Twilight hit the ground with a dull thud, their eyes unblinking. S-169 prodded their fallen bodies, confirming the kill before stepping through her portal, which closed as soon as she was through the threshold.

Twilight Sparkle scribbled on her chalkboard, moving through the equations lightning-quick. Her magic made scribbling her formulae easy, and truly made her appreciate how difficult it must be for earth pony scientists to get anything done. Once she was done here, she’d have to look into ways to make things easier for them. But that would come later. Currently, she was about to uncover the greatest scientific discovery of all time.

She hadn’t seen her friends in a few days, but they understood. They knew how Twilight could get whenever she had her mind set on something, especially something so important. So, they gave her space, knowing Spike would pry her from her numbers and calculations eventually. But, for now, they would let her have her obsession peacefully.

“You should take a break, Twilight.” Twilight hummed curiously, looking away from her numbers to see her number one assistant waddle into the lab, clutching a tray of tea and sandwiches. She had been far too busy to notice the loud, animalistic growls of her stomach. Spike chuckled, having long since been used to seeing Twilight lock herself away, starving and dehydrating herself until she reached a breakthrough. So it was with a measured practice that he cleared off a section of Twilight’s desk and sat down his tray of tea and sandwiches.

“Hey, Spike. I was just working on this spell,” said Twilight. “I feel like I’m missing something…”

“You are. It’s called lunch,” said Spike. He poured out a cup of tea just the way she liked it; Three sugars and a bit of lemon. Twilight nodded her appreciation before tearing herself away from her board and taking a seat beside Spike. “Come on, Twi, you’ve been cooped up here for two days now. You need food.”

“Sorry, sorry, you’re right,” said Twilight. She trotted over to her friend and took the cup he offered. She took a sip, sighing happily as the warmth enveloped her body. “I’ve been so focused on this frequency. The math, the science, it’s all very frustrating. Sometimes, I feel as if the numbers are changing, as if the very fabric of the universe is shifting to evade me.”

“That’s probably you going delirious from lack of food and sleep!” said Spike. “How are you gonna keep changing the world if you’re too hungry and tired to think straight?”

“Ah, you’re right. As usual.” Twilight levitated a sandwich from the plate and took a bite. “I think it’s about time I take a little break. I owe Rarity a spa trip, and I may as well make it a group affair.”

“Good. You’re gonna drive yourself crazy cramped in here.”

There was an ear-splitting squeak, like the shrieking of rending steel.The air in front of Twilight’s chalkboard began to shake and shimmer, then split into a void of swirling violet light. Spike and Twilight backed away, mesmerized by the swirling smoke that emanated from it.

“What in Celestia’s name…” Twilight backed up, spreading one of her wings out to block Spike from any harm that might come through.

To her complete and utter shock, it wasn’t harm that came through the portal, nor was it some unknown horror. What came through the portal was… Twilight Sparkle, except it wasn't quite. Her eyes were bluish and her mane was cut a bit shorter but, save for those small differences, it was like looking into a mirror. The doppelganger wore some sort of band around her hoof, steel with a glowing blue button right in the center, and saddlebags on her back, which she wiggled out of and set on the ground.

“Is this the right place?” asked the newcomer. She looked around, taking survey of the area. “Alicorn Twilight, drake Spike, cluttered lab… Oh, this is a castle, hm? Shame. I miss that old tree…”

“What in the wide, wide world of Equestria!” Spike pushed down on Twilight’s wing to get a better look at… Twilight.

“Ah, Spike. Or, should I say, Spike C-317!” said the new Twilight. “Hello! I’m Twilight Sparkle, though I believe it’s safe to say we’ve met in some capacity.”

“You're… You're me…” said the original Twilight, utterly perplexed. “How is that possible?”

“Technically, I'm not you. I'm me,” said the new Twilight. “And you're you. But we're both Twilight. We're exactly the same, except for where we're different.” She tilted her head slightly. “Does that make sense?”

“Not at all.”

“Good. That means you're a sane one.”

“Twilight, have you been messing with time travel again?” asked Spike. “This is crazy!”

“Ooh, you're on time travel already? Most of us don't get there this soon,” said the new Twilight. She shook her head firmly, bringing herself back to the matters at hand. “Sorry, I'm getting off topic. Allow me to properly introduce myself. My name is Twilight Sparkle, from Equestria X-215.”

“Equestria X-215? What does that even mean?” asked the original Twilight.

“Hm… Those numbers are a bit wrong,” said Twilight X-215, inspecting the board. “You're on the right track, though. I'll help you work it out when we're done.”

“You've cracked the secret of interplanetary teleportation?” asked Twilight. She shook her head. “What am I saying? You just stepped out of a portal, for heaven’s sake!”

“Interplanetary? No wonder you're struggling. That's not going to give you Interplanetary travel. It's going to give you interdimensional travel.”

“Uh… What?” asked Spike.

“May I?” asked X-215, pointing at the chalkboard. “It’ll be easier to explain with visual aids.”

“Um… Sure?” said Twilight uneasily. X-215 nodded, then grabbed an eraser in her magic. Curiously, her horn glow was a deeper color than Twilight’s. She quickly wiped the board clean, then drew a small circle in the center of the board.

“What did you have for breakfast today?” asked X-215.

“Nothing. I skipped today.”

“Of course.” X-215 wrote “N/A” above the circle. “Did your Spike offer you anything?”

“Eggs and oatmeal.”

“Typical enough. Well, you ate nothing today, but you also ate eggs.” X-215 drew another circle next to the first, labeling it “Eggs”. She then drew one more circle, labeling it as she spoke. “And you also ate oatmeal. And cereal. And soup. And glass.” She drew a circle to coincide with everything she listed, connecting them to the original circle until it resembled a diagram of some sort of solar system. “This is you.” She pointed to the original circle. “This, the eggs circle, is me. Any questions?”

“Several,” said Twilight. “Why do you keep calling him ‘my’ Spike? There’s only one Spike.”

“Wrong. There’s only one Spike here. But I have mine from home. And Oatmeal has one. Cereal has one. Glass has one. It’s one of the only constants,” said X-215. “Where there are Twilights, there are, were, or will be Spikes.”

“I’m… I’m confused…”

“Which is good. I told you, it means you’ve got your head on straight.” X-215 erased the board, starting a new diagram. This time, she worked from left to right, drawing a series of boxes. She started with one, then two, then four, eight, sixteen. “If you understood this straight away, that would mean you’re one of the mad ones. Which, besides being incredibly upsetting, would be a waste of my time, and I don’t exactly have a ton of that to waste.”

“But you have enough to waste on breakfast metaphors?” asked Spike.

“First of all, it’s not a metaphor. Second of all, yes, because I need you to understand.” X-215 pointed to the first box. “This is Twilight Sparkle. Twilight Sparkle is about to make a decision; breakfast or no breakfast.” She pointed to the second box. “This is Twilight A. She ate breakfast. The one below her is Twilight A-2. She didn’t. Twilight B had eggs. B-2 had oatmeal. C took an early lunch. C-2 didn’t eat until dinner. And so on and so forth.” X-215 erased this diagram and replaced it with another, this one a massive mess of dots and lines connecting and crossing one another. “This is you.” She pointed to a dot amongst the mess. “This is me.” She pointed to another dot on the other side of the diagram. “Every time you make a decision, you are creating two possible yous; One that did the thing and one that didn’t. Every single possible thing that could happen is happening, has happened, and will be happening. I come from a world that unlocked the technology required to open little doorways between these possibilities. I’m a you from a time you went for a salad instead of a burger at a restaurant four years ago. Or something. Honestly, it’s not that simple, but for the sake of this discussion… Are you absorbing all of this?”

Twilight’s jaw went slack as she tried to take in all of this information. It was insane. The ramblings of a mad mare. Except… the mad mare in question was herself. If Twilight couldn't trust Twilight to tell the truth, who could she trust?

“So… There are how many of these other realities?” asked Twilight.

“Oh boy, leave it to you to ask the tough questions,” sighed X-215. “Theoretically, there's an infinite number of universes. But, there also have to be universes in which there are only one. We haven't been able to confirm that constancy. But the number of them don't matter. What matters is the number of Twilights.”

“What do you mean?”

“Twilights are dying. Murdered,” said X-215. “And I need your help to stop the killer.”

“K-killer?” said Spike nervously. “Who would do something like that?”

“Me. Her. Us.” X-215 rummaged through her saddlebag, producing two steel bands like the one she wore, one of them resized to be much smaller. “A rogue Twilight. And, while she’s on the loose, none of us are safe. Here, take these and put them on.” Twilight and Spike uneasily took them and slipped them on.

“Why would a Twilight kill other Twilights?” asked Twilight.

“We’re not sure. She could be one of those mad ones I was talking about. But that doesn’t matter. Remember those constants I told you about? Well, one of those constants is that all Twilights are clever, determined and, under the right circumstances, dangerous.” X-215 press the button on her band, which projected a panel of light that she punched a few numbers into. “I’m calibrating your Containers now. We’re going to the latest victim to find some clues.”

Twilight glanced at Spike, who she could only imagine to be freaking out. This was a lot to take in for anyone. Twilight was a certifiable genius and even she had issues grasping the concept. She let out a sigh, then walked over to join her counterpart.

“Spike, stay here. It’s too dangerous,” said Twilight.

“Wrong,” said X-215. “The safest place in all of the multiverse for a Spike is right next to his Twilight. If the rogue ends up here, she will take him and use him for leverage to get whatever it is she wants.”

“I b-better go with you, then,” said Spike uneasily.

“Just be careful,” said Twilight. She wasn’t happy with the situation, but there wasn’t much that could be done for it. X-215 needed her help, and, if what she said about the rogue Twilight was true, then everyone was at risk. “Um… Twilight? I’m ready.”

“You can call me Twilight X-215, to avoid confusion,” said X-215. She thought on it for a bit, shaking her head. “That’s a mouthful, isn’t it?”

“How about TX?” suggested Spike.

“I like that. Alrighty, then. Twilight and Spike of C-317,” said TX. “TC, SC. Be prepared. What you see when passing through this portal will be horrific. It will be hard to wrap your head around. But, once you do, the understanding, the knowledge… It’s beautiful.”

TX stepped through the portal, disappearing through the void. Spike clambered onto Twilight’s back, holding on tightly. Twilight glanced at her board, the diagram of the many realities, with trepidation, then cautiously followed her doppelganger into the portal.