>> Change.
Time is like a river. If you make a little change upstream, the changes downstream can be both meaningful and unforeseen. Of course, in both cases there is also a chance that a little change upstream will be eroded away by the end, making the end result unchanged. In both cases it is far more widely observed for there to be great changes, but this could be an example of observation bias. After all, if no great change occurs then we never think to look. The only way to be sure is to follow the river to its end. <<
When Twilight asked Starlight to bring a friend to the dinner party Starlight was torn. On the one hoof it would be easiest to ask one of the Time Ponies to join her, but on the other hoof they might bring up the project. But after failing to find a new friend to bring with all 5 of Twilight's friends, Starlight was ready to bring a friend from their ranks. Now she sat by the fountain deciding who to ask. Minuette was friendly enough, but had no filter whatsoever. She would be sure to let slip their secret project. Perfect Pace had headed back to Canterlot to attend to his clock store. That left Time Turner and Derpy.
“I know how to decide. I'll ask Derpy if she knows what we've been doing. I don't think anypony has told her the details, which would mean she can't accidently tell Twilight. It's perfect.” Since Starlight didn't know if Derpy had work this Saturday she might as well start looking for her at home. Besides, if there was one thing Starlight had learned about Derpy in the past month, it was that she took her job as mailpony very seriously. Better to wait and ask once her shift was over if she had work today.
Once she reached Derpy's house she knocked on the door. A gray unicorn stallion with a purple mane cracked the door open. “Hello, may I help you?” he asked.
“I'm here to talk to Derpy, is she home?”
“I'm in the kitchen! Feel free to sit on the couch!” Derpy answered. The stallion opened the door the rest of the way and stepped aside to let Starlight pass. She walked into the house and walked over to a wall with pictures hanging on it.
Before she got a good look at the pictures the stallion cleared his throat and she turned to face him. “Hello Ms., my name is Written Script. May I ask for yours?"
“I'm Starlight Glimmer. I live in Twilight's castle.”
“How do you know my sister?”
“I met her at the post office and she took me to see Time Turner.”
“As a friend, not a mailmare!” Derpy said as she walked into the small entryway and living room. “I always remember, ‘the Ponyville Post has a strict policy not to share anypony's address’. But Doc said I could show anypony where he lives if they seem nice, so I took her home after work.”
Then two foals walked in behind Derpy and stood on either side. “Hello Ms.,” said the unicorn filly. “Would you like a muffin?”
“No thank you,” replied Starlight. “I just wanted to talk to Derpy about some things.” She looked at Derpy. “Do you mind if we step outside for a minute?”
Script raised an eyebrow but said nothing and Derpy shook her head. “Nope! As long as it won't take long, I want to be there when we take the muffins out.”
“This should only take a minute,” said Starlight, leading Derpy into the front yard. She looked around for anypony who might be watching, but it seemed clear. “Do you know what I'm working on with Time Turner?"
“Yep! He told me that when he asked me to send the letters the day after we met. But don't worry, the secret is safe with me. ‘Mailmares never give out personal information about their clients.’ I'd sooner give up muffins then break the Postal Code.”
“How many secrets do you keep?”
“I can't tell you that. I just told you, ‘Mailmares never give out personal information about their clients.’”
“Derpy, how would you like to come to a dinner party with Twilight and Celestia at the castle tonight?”
“That sounds great! When should I come? And which castle?”
“I'll come pick you up at six. Have fun with your family!” Starlight and Derpy waved and headed off in their separate directions.
Derpy walked back into her house where some of her siblings were waiting for her. Her big brother asked, "What was that all about?”
“She invited me out to dinner, isn't that nice?”
“What did you say?” asked Dinky and Ray.
“I told her that sounded great.”
“Where did she ask you to?” asked Script.
“She asked me out to the castle.”
“Do you want me to come with you?”
“No, I think she wanted me to come alone. She said she would pick me up here at six.”
“Dinky, Ray, go play outside, Derpy and I need to talk."
The other three seemed puzzled as to why the foals had to go for the adults to talk, but outside was fun so Dinky and Ray left without complaint. “What do we need to talk about?” asked Derpy.
“I thought you were seeing Time Turner? Why are you going out with Starlight?”
“Because Time Turner is busy tonight and I like Starlight. We'll have a wonderful evening and I can be with Time Turner tomorrow. There’s nothing that says I can only go out with one pony.”
“Alright, as long as you’re sure. Come see me if anything bad happens at dinner.”
“I will. You worry too much, it'll be great!” Derpy looked out the window. “It's time to come in and check the muffins!”
At about five Amethyst Star came home to find Derpy sitting in their front yard. “Hey sis, what's up?”
“I'm waiting for Starlight to pick me up at six! We're going to have dinner at the castle with Twilight and Celestia!”
Amethyst walked inside and looked at their wall clock. It read five fifteen. “You have half an hour to get ready, are you planning to wear anything in particular?”
“Well, I wasn't. Do you think I should wear my postal outfit? I like it, but I thought this was a casual dinner.”
“With not one, but two princesses? Why don't you borrow my skirt? It matches your mane and is just right for a party, regardless of whether it is a casual or fancy one. And I'll brush your hair too. It wouldn't do to leave it tangled.”
“Thanks! I don't know what I'd do without you.”
“Don't mention it. After all, what are sisters for?”
“Baking muffins together?”
“Ahh. Yes Derpy, that too.”
On Starlight's way to pick Derpy up she was stopped by Written Script stepping into the road ahead of her. “I want to talk to you about tonight,” he said, pointing to the side of the road.
She stepped off where he had pointed and said, “Why's that?” Please don't know about my past. Please don't know about my past.
“I don't want to see Derpy hurt. I don't know what your intentions are, but remember, treat her well.”
“I'm not going to hurt her! I don't do that anymore, I promise!”
“You'd better not,” he said, walking past Starlight and turning off the street out of sight.
Well that could have gone better. I guess they really do know about my past. She sighed. At least Derpy was willing to forgive her.
She continued on her path until she got to Derpy's house. Derpy was standing in her front yard in a little yellow skirt. When she saw Starlight she waved and trotted over.
“With your skirt suddenly I'm wondering if I should have found something to wear after all,” said Starlight with a chuckle.
“Is it too much? I wasn't sure, but Amethyst said it would be perfect…” Derpy looked down at the skirt.
“It's lovely,” Starlight reassured her. “Just don't bring up the project and it'll be fine. I already told Twilight I was bringing you and she seemed pleased. Relax, you don't have to do anything but be yourself.”
“Really?”
“Really. I even made sure we had muffins on the menu.”
Derpy hugged Starlight “Thank you!”
“Now let's get going before we're late.” Starlight started walking towards the castle.
“Yes ma’am!” Derpy said while bringing her right hoof up, nearly but not quite touching her head.
“Woah! What was that?”
Derpy brought her hoof back down and started following Starlight. “What was what?”
“That thing you did with your hoof.”
“That was a salute! A salute is how a soldier respectfully hails a superior officer, or occasionally a government official, such as a mayor or noble.”
“Okay.” They continued on their way to Twilight's castle in silence.
Written Script watched as Starlight led his sister into the castle. He knew he couldn't just walk right in and keep an eye on them, but there was a way he could observe them. He didn't like Starlight, and suspected Derpy might not fully understand the implications of somepony asking her to their residence for dinner alone. And the fact she admitted that she used to hurt ponies made him worried. Just what had Starlight done before, and was she truly past it?
He made his way from a bush to the back wall of the castle and put his ear to the wall. He didn't hear anything, and nopony could see him on the outside, so he grabbed the ladder from a bush under one of the windows where he had stashed it that afternoon. He set it up with his magic, cautiously made his way up to the window, and climbed through. Then he used his magic to fold the ladder back up and return it to the bush.
Once inside he looked around. He was in a hallway stretching in both directions straighter than the outside implied was possible. This could be a tricky search, but surely no harder than navigating Celestia's palace. He set off to the right and listened for anypony.
Starlight and Derpy arrived right on time. They had just sat down when Celestia arrived. Starlight bowed respectfully while Derpy gave another salute. Celestia chuckled and returned Derpy's salute. Only after Celestia had put her hoof all the way back down did Derpy lower her own. “It has been some time since anypony has saluted me. Normally they either bow or simply offer a greeting. May I ask why you chose that particular gesture?”
Derpy began another salute. “A salute is how a soldier respectfully hails a superior officer, or occasionally a government official, such as a mayor or noble. As a member of the E.U.P. reserves I salute you, Princess Celestia, Solar Diarch of Equestria, as the commander of all military ventures, should you require to do so personally.” Celestia returned her salute and Derpy lowered her hoof once more.
Twilight gaped. “Derpy, that was a rephrasing of section 12.3 of the E.U.P. official guidebook. Did you memorize it?” While Twilight and Derpy talked Starlight and Celestia started eating.
“No. I forget pieces of the book sometimes. That's why I make sure to read through it and the mailpony guidebook at least once a year.”
“Now I remember! You're the one who was always checking out the military history books. If you'd like you can check them out of the castle.”
“Really?”
“Of course. I've never officially opened up my private collection to the public, but any of my friends can borrow anything they want. And any friend of Starlight's is a friend of mine, right Starlight?”
“What? Oh, yes. The more the merrier! He he he.” Starlight thought back to Time Turner. She didn't like hiding anything from Twilight, but she couldn't tell her about the problem.
“Yay! I can reread the complete history of the Wonderbolts! I thought I had lost it forever when Tirek destroyed the library. Thank you so much!”
“So,” asked Twilight, “how long have you been in the E.U.P. reserves?”
“Ever since I turned seventeen. That's when I was old enough to sign up. I wanted to try out for the Wonderbolts, but I failed the medical exam.” Derpy frowned and looked down, her eyes drifting apart. Then she looked up and smiled, “But I passed the E.U.P. reserve test! Virtute Alisque!”
Written Script was about to give up and go home when he heard Derpy shout up ahead. It sounded happy, so he crept carefully forward. Soon he came to a large green stained glass window. He peered through and saw Princess Celestia sitting across a circlular table from his sister, with Princess Twilight on one side and Starlight on the other. He saw them share a laugh and he relaxed. Maybe Starlight really wasn't so bad. Now he just had to leave without getting spotted and all would be fine.
He turned around and saw Princess Twilight's assistant Spike looking at him. “Why are you sneaking around Twilight's castle?”
He sighed, “I misjudged Starlight. And I think I misunderstood my sister."
“Your sister?”
“Derpy. She said she was going to have dinner with Starlight, and I jumped to conclusions. I don't know what Starlight did, but I guess it really doesn't matter. She regrets it and doesn't do it anymore, plus Derpy trusts her. She seems to be a good pony. Would you tell her that Written Script sends his apologies? I'd tell her myself, but I don't want to interrupt the evening, and I should probably go.”
“Alright, I'll be sure to tell her. And next time just knock on the front door, it's always open.”
Written script nodded and went home.