“Oh no, oh no, oh no! How could I let this happen?!” Twilight moaned as she raced through the streets of Ponyville, Spike riding calmly on her back. “It's been over a month since I left Canterlot and I haven't gotten in touch with Sunset once!”
“To be fair, you have been pretty busy,” Spike pointed out. “Defeating Nightmare Moon,” he began to tick off on his claws, “learning about friendship, getting settled into Ponyville... You've had a lot to do lately. I'm sure Sunset will understand.”
“So?!” Twilight exclaimed, shocking a few of the ponies she was running past. “I'm pretty sure spending all your time getting to know new friends, at the expense of your old friend, makes you a bad friend!”
They skidded to a stop in front of the train station, and Twilight immediately began to dance anxiously on her hooftips. “Ohhhh, she's gonna be so upset!”
“Maybe it won't be so bad,” Spike said as they waited. “I bet she'll even be proud of you!”
“You'd better be right, Spike,” Twilight replied, rushing headlong into the first car that opened once their train arrived.
The trip from Ponyville to Canterlot wasn't even an hour long, but for Twilight it felt like an eternity. Those unlucky enough to be sharing the space with her felt similarly, as she spent the entire way there in a state of worry and panic that Spike was barely able to keep under control. It surprised nopony when she was the first to rush off the train, assistant clinging to her, when they finally reached their destination.
Through the city streets at full gallop to the School for Gifted Unicorns, past all their favorite spots on campus – empty, of course – and finally up the steps to Sunset's home. As Princess Celestia's personal student, she'd been given lodging in a tower not too different or too far from where Twilight used to live, and it was there that Twilight hoped to find her friend. With a breath of caution she took hold of the door with her magic, and pushed it open.
It was completely dark; even the curtains on the windows had been drawn.
“H-hello? Sunset?” Twilight nervously called into the spacious apartment. “It's me, Twilight! I...” She swallowed. “I'm back!”
No answer came, so she took a few tentative steps inside.
Then, all around her, candles flared to life with such force that pony and dragon alike recoiled in fear and screamed. And when they finally realized that fiery death wasn't imminent, they heard the telltale sound of hoofsteps on wooden stairs.
A door was flung open nearby, prompting another short scream. Light spilled up and out of the stairwell behind it, but even shadowed they could still make out the form of Sunset Shimmer.
“Well. Well. Well,” she opened, enough ice in her voice to stand up to the summer sun. “Look who finally remembered me. What, did you already get bored of your Ponyville friends?” she asked, sneering as she spoke the last two words.
Her horn lit up as she slammed the doors and stepped forward. Even in the candlelight Twilight could see the deep bags under her eyes, the messiness of her mane and tail, and a variety of stains and scuffs on her coat. Either she hadn't been sleeping well, or she'd been doing some demanding experiments. Probably both.
Twilight was frozen by her glare for some time before she finally spoke up. “Sunset, I'm so sorry. I never meant to be away for so long! Please, let me explain!”
Sunset snorted, opening the curtains and snuffing the candles with her magic. “Spare me, Sparkle. I already know what happened in Ponyville. You made five new friends,” she said, rolling her eyes and a hoof, “harnessed the powers of the Elements of Harmony, and turned Nightmare Moon back into Princess Celestia's sister.” She gave Twilight a withering look. “And every few days since, you've sent the princess a letter about what you've learned about friendship.”
“I... should have expected that you'd already know about all that,” Twilight admitted with a sheepish, shamed laugh.
“Yeah,” Sunset told her. “But instead of learning about it from you, like I should have, I had to learn it from the Princess when she arrived with her sister in tow.”
Twilight shrunk under the ferocity of her glare. “S-sorry...”
“You should be.”
Spike gulped and slid off Twilight's back. “Umm, I'll just, uh, go catch up with Moon Dancer and the others...” Neither mare gave notice to his nervous words, nor him scurrying out of Sunset's apartment.
“Do you even realize how worried I was when I found out that you were right?” Sunset asked, averting her gaze just a moment to hide the tears she was forcing back. “Knowing that you'd try to stop Nightmare Moon by yourself if you had to? Knowing how much danger you'd be putting yourself in?”
“I, I'm sorry,” Twilight stammered, tears coming to her eyes too, “I didn't-”
“Do you realize how overjoyed I felt when I heard that you'd saved the day?” Sunset asked, doubling down on the force in her voice. “How proud? I didn't even mind it when I heard you'd be staying in Ponyville longer, because at least you'd still be sending letters!” She was on her hindlegs now, thrusting her hooves into the air. “But not once did you mention me in them! Send me a message! Ask how I was doing!”
“I'm so sorry...”
Sunset fell back on all fours, her tears now unrestrained now matter how hard she sniffed. “Having Princess Celestia be too busy reconnecting with her sister to spend time with me is one thing,” Sunset told her. “But having the only other pony I've ever cared about suddenly forget about me... it hurt, Twilight!”
Twilight winced. “I just...I just got so caught up in learning about friendship,” she said, looking down in guilt and shame. She sniffed, then put on a sad smile and said “You know how I get with new research... The... The 'Twilight Zone'?”
Sunset closed her eyes and shook her head. “It's hard to like that about you when I'm the one being pushed aside.” She huffed bitterly, then looked aside and muttered “But what else is new...”
This got enough of Twilight's attention to pause her tears. “What do you mean?”
She was answered by a short, bitter laugh. “I mean that I keep getting pushed aside for somepony else, and I'm tired of it. Why do you think I hated you so much when we first met? Princess Celestia is the closest to a mother I've ever had, and I thought she was replacing me because I wasn't what she wanted.”
“But Princess Celestia would never do that!” Twilight protested out of shock. “She loves us equally!”
Sunset snorted and muttered “I doubt it.” She saw Twilight's questioning look, and continued. “Before she left for Ponyville that day, she told me and Cadance and your brother in private what was going to happen and what we'd have to do in case of the worst. But most of all, she told us to trust you.”
“What's wrong with that?” Twilight asked, curious yet afraid.
“She told us why,” Sunset answered bitterly. “I didn't realize it then, but after thinking about it it's obvious that she knew from the beginning that you were pretty much destined to save her sister.”
Twilight's eyes widened. “Destined...?”
“She made you her student the moment she realized how special you were,” Sunset continued without pause. “And did you know she made Cadance a princess and even adopted her after only knowing her for like two days?” she added in maddened disbelief. “And here I am, her other student, with nothing. Nobody cares about Sunset Shimmer, oh no,” she said, throwing her hooves up again, “she's just a bad-tempered disappointment.”
She wiped away her tears, but they kept coming. “All. I ever wanted. Was to matter,” she said through clenched teeth. “To be special to somepony.” She turned her back on Twilight, and said “But I'm obviously not.”
She began to walk away, but only got one step before Twilight darted forward and nuzzled her. “You're special to me, Sunset Shimmer.”
“Clearly I'm not if you forgot about me for a month...”
Twilight was silent for a good long while before she finally said “I wish I could say that I didn't. But I came here the moment I remembered, and I don't want to ever forget about you again.”
Sunset scoffed. “You say that now, but once you're back in Ponyville with your new friends...”
“Then come back to Ponyville with me,” Twilight said, walking around to Sunset's front. “I won't be able to forget about you if you're always around, right?” she said, offering another sad smile.
“You... you're joking, right?” Sunset asked, eyes widening in shock. "I chew you out for abandoning me, and your response is to ask me to move to Ponyville with you?"
Twilight nodded, offering her friend a wan smile. “I have to make up for what I did somehow, don't I? But I can't leave Ponyville right now, and you seem like you're, well, miserable here. So if us being apart is the problem, then why not come together? You can stay in the library with me and Spike, there's plenty of room. And I'm sure Princess Celestia will let you continue your research there.”
Sunset let out a weak laugh, wiping away some of her tears. “Yeah, probably. But...you're really serious about this?”
“Yes,” Twilight nodded. “I want my best friend back.” She paused, blushing as she added “If she'll have me, that is.”
Sunset just kept staring for what felt like ages before she closed her eyes, shook her head in disbelief, and replaced her shock with a familiar smirk. “You know what? I'll think about it.”