"So let me get this straight..."
Derpy was hovering back and forth in the large main chamber of the TARDIS, her hoof on her chin and a thoughtful look on her face.
"You have a magic outhouse..."
The Doctor sighed from behind the sonic screwdriver he was using to inspect the lock on the TARDIS' door.
"That can travel through time and space?"
The Doctor looked up from his work on the door. "For the third time, Derpy, yes! You've seen it in action! Now do you want to travel with me or not?"
Derpy had, indeed, seen it in action. After she had stumbled into the TARDIS the day before, the she and the Doctor has traveled to New Equestria to attend the grand opening of New Sugarcube Corner, run by none other than the great-great-great-great-great (etc.) grandchildren of Ponyville's own Mr. and Mrs. Cake. When they had returned, the Doctor has tentatively extended an offer for Derpy to join him in the TARDIS.
He wasn't sure why he had offered. She definitely wouldn't make the best traveling companion he had ever had. She was clumsy and absentminded, and the universe could be a dangerous place. There was just something about her that he couldn't place a hoof on.
"Sure! Ohh, where are we going next, Doctor?"
The Doctor stopped scanning the door with his screwdriver and checked the results. Again, they were disappointing.
"Absolutely nothing! How can there be absolutely nothing wrong with this door?" the Doctor thought. "Nothing can get into the TARDIS, it's the most secure place in the universe!"
The Doctor put the sonic screwdriver back in his collar with a sigh. This was another thing that had influenced the Doctor's decision to invite Derpy to travel with him. Anypony who could simply walk into the TARDIS would need to be watched carefully. Either they were very dangerous and resourceful, and he had to keep an eye on them to make sure they don't start any trouble, or...
"Doctor!"
The Doctor flinched, surprised out of his musings by the grey pegasus who was on his mind. "I'm sorry, Derpy, my mind tends to wander after that incident on Curunir... now that I think about it, I never did get those brain cells back, did I?"
Derpy giggled. "So where are are we going next?"
The Doctor smiled. This was one of his favourite parts.
"Anywhere you want, my dear!"
The Doctor sighed again and facehoofed. "No, Derpy, think bigger."
The mare in question hummed thoughtfully. "Okay, if you won't take me to Cloudsdale, then how about..."
The Doctor braced himself for another less-than-exotic destination request from Derpy. For the past ten minutes, she had been suggesting every city in Equestria she could think of, from Cloudsdale to Hoofington. She didn't seem to understand the concept of 'time-and-space travel', no matter how much the Doctor explained.
"The Castle of the Royal Pony Sisters, before Nightmare Moon!"
"Derpy, think bigge- wait, what did you say? Hmm, that actually sounds quite interesting..."
The Doctor jumped up and ran to the console that dominated the center of the room. "Ah, the first capital of Equestria! Built from the ground up by the magic of Celestia and Luna! I haven't been there... well, since it was built! I hope I'm not still wanted..."
Derpy followed the Doctor to the console and watched as he started pressing seemingly-random buttons and pulling levers. "Wanted? Why, what did you do?"
"After a string of misunderstandings, they thought I was Discord in disguise," the Doctor explained. "To this day, I still don't know how that banana grew legs and assaulted the princess."
The large column in the center of the console started oscillating back and forth, and the familiar whooshing sound the ship emanated around them. The ground started to shake, and Derpy had to jump up and hover in order to not be thrown around.
After a few moments, the movement stopped and silence fell over the ship. The two made their way to the door, and the Doctor stopped Derpy before she went any further.
"Before we head out there, there are a few rules you'll need to follow," he said, a serious look on his face. Derpy nodded. "First, no interrupting established events. Time has ways to fix itself, but I'd rather not ruin the future or create a paradox. Second, if you see either yourself or me out there, do not speak to yourself! Keep your head down and try not to be noticed."
Derpy scratched her head, a confused look in her already confused-looking eyes. "Why would I see myself out there? I'm not even born yet!"
"Perhaps in our future travels, we'll return here. Maybe something timey-wimey goes wrong." The Doctor shrugged. "It's not likely to happen, but it's a possibility. Now! Are you ready?"
Derpy nodded, eager to see the past with her own eyes.
"The first capital of Equestria! The castle of the Royal Pony sisters! One of the happiest time in Equestria's history!" The Doctor went up to the door and placed his hoof on it. "I present to you..."
The door swung open.
"...the past!"
Derpy flew out of the TARDIS with an excited squeal and the Doctor followed.
In front of them, the tall castle rose into the sky. Banners and flags flapped in the wind, and the central spire of the castle was elegantly framed by the sun placed perfectly behind it, almost as if it were placed there solely for the effect.
Derpy looked up in amazement, entranced by the sight. The Doctor, however, was inspecting other things. Behind them, what should have been a bustling marketplace in the center of the city was empty. No vendors were selling produce from their stalls. No foals were playing in the street, being watched over by their parent sitting in the shade of the grey-stone buildings.
"Derpy..."
"It's beautiful!" Derpy exclaimed. "Thank you so much for bringing me here!"
"Derpy, come here, please."
"Oh, it's one of the most amazing things I've ever seen!"
"Derpy!"
Derpy turned around and hovered over to the Doctor's side. "You didn't have to shout..." she grumbled.
The Doctor ignored her. "There's something wrong. This market should be bustling with activity."
Derpy looked around, surveying the empty city streets. "Maybe it's a holiday?" she suggested, scratching the back of her head.
"It shouldn't be, not today. Besides, even if it were, there would still be some ponies out and about. It's absolutely deserted..."
The Doctor turned towards Derpy. "I think we should investi-"
His sentence was cut short when he turned around.
Derpy was gone.