//------------------------------//
// Exposition Express 
// Story: Bringing humans to Equestria 
// by chaotic serenity 
//------------------------------//

Derpy was confused. She had never heard of a human before. She never paid much attention in school, but the way the Doctor said human worried her. They sounded dangerous, and she always paid attention when the teacher was talking about dangerous animals.

"I don't understand." she said. "What's a human? How did it get here?"

"I'll explain later," the Doctor replied. "For now get him into the TARDIS. I need to run some tests."

The Doctor grabbed one part of the human and started dragging him towards the TARDIS. Derpy grabbed the top part of the human and began to help the Doctor, wishing the entire time that she was a unicorn. They took it to an area of the TARDIS Derpy had never seen before and laid it down on a bed. The room had just one bed, one entrance, and a window with a one way mirror for observation.

"There we are." the Doctor said when they were done. They walked outside and the Doctor locked the door. As Derpy watched, the Doctor pressed some buttons on a panel just underneath the one way mirror. The lights inside the room turned red and something began to hum.

"Doctor? What is that room?" Derpy asked, "It isn't hurting the human is it?"

"Of course not Derpy!" the Doctor exclaimed. "This is a medical room. This equipment will scan him and treat him if he has any injuries. I've also set it to keep him unconscious until I can figure out what to do next. Now let's get back to the main console room."

They began to walk towards the console room, the Doctor being unnaturally silent. Derpy had a million questions she wanted to ask, and wanted to ask them all at once and was frustrated that she couldn't. She had tried that once, and it got her sent to the principals office because the teacher thought she was swearing. Derpy knew she had to ask something or she would explode. So she decided to ask the first thing that came to mind.

"Doctor?" she began, "How can you tell if the human is a male?"

"I used to live with the humans." the Doctor said. "It would be very embarrassing if I couldn't tell between male and female."

"Oh."

Derpy nodded and continued to follow...wait what? Derpy paused in the hallway under the realization of what the Doctor just said. He used to live with the humans? Now Derpy had even more questions to ask and it was making her head spin. After looking up and realizing the Doctor hadn't notice her stop, Derpy hurried along after him. Eventually they made it to the console room, and the Doctor began to look over the various scanners and controls.

"So..." Derpy began after a few minutes of silence, "When you say you used to live with them..."

The Doctor looked up and sighed.

"I suppose I should tell you." He adjusted his neck tie and began to tell his story. "I wasn't always a pony. In fact, I didn't always live in this universe."

Two sentences in and Derpy was already confused. What did he mean he wasn't always a pony? What else could he be? She knew he was something called a time-lord, but she just thought that was a different kind of pony. She let the Doctor continue to speak.

"I told you how that creature came from a different dimension. Well, I used to live in another dimension. One without pegasi, earth ponies, or unicorns. In fact, I used to look like that creature in the medical room."

"But then...how did you get here?" Derpy asked.

"I fell through a crack in the void." the Doctor replied. "It was a small thing. Barely noticeable, yet big enough to suck the TARDIS into it. Normally, anything that gets sent into the void is lost forever, but the TARDIS has a failsafe in case that happens. It locks onto the nearest crack in the void, disregarding the one it fell into, and goes through it."

Derpy nodded, but she only understood part of what the Doctor was actually saying. She was still having trouble accepting that the Doctor hadn't always been a pony.

"I had fallen unconscious during the transition, as the TARDIS was thrashing about so much. When I had come to, I was a pony. I made my way through the doors, and that's when I met you Derpy."

Derpy nodded in rememberance. She flashed back to the day she had met the Doctor. She was taking one of her usual strolls through the forest and came across a blue box. Just as she flew up to it, the doors flew open and out popped the Doctor.

"Doctor," said Derpy, "Could the human have fallen through this 'void' thingy like you?"

"No, impossible." the Doctor said. "Those cracks are so far apart and usually so small that nothing gets sucked in, unless the crack is forced open. The one I fell through was an anomaly."

"So how did the human get here?" Derpy asked.

"Well, remember those blue buttons?" the Doctor said. Derpy nodded. "Those were controls that allow the TARDIS to travel between worlds. However, the only way to safely cross between dimensions is to have some sort of anchor on the departing side in order to pull you back when you want to go home. Usually that means another TARDIS but..." the Doctor paused and for a brief moment Derpy saw a flicker of sadness pass over his eyes. "Anyways," he continued, "Trying to cross without an anchor is like trying to fight the tide."

Derpy was glad for the analogy, as she was having trouble keeping up with the Doctors explanation. She often felt that the Doctor sometimes forgot that not everypony was as smart as he was.

The Doctor continued, "When you pressed those buttons, the TARDIS tried to cross dimensions. However not only did we have no anchor, we also had no destination. So the TARDIS picked a spot at random and tried to jump. However the 'tide' of the void quickly shoved us back, but not before we made a small impact."

"What do you mean?" Derpy said, suddenly feeling uneasy. "What kind of impact?"

"For the briefest of moments," The Doctor explained, "The TARDIS landed in the other dimension. But it wasn't complete. Most of the TARDIS had materialized in that world before the void sucked us back to Equestria. But the TARDIS is very powerful, and cracked the void on that side. If I'm correct, and I'm always correct, the crack started there and led here. Since the crack was forced, it began exhibiting a black hole effect. Anything that was close by when the TARDIS landed, was brought back to Equestria with us. It just so happened to be that human."

"So, we caused him to follow us?" Derpy asked. The Doctor nodded and looked up at the scanner. He seemed to be thinking about whatever the scanner was showing him. Then he looked back to Derpy.

"It's worse Derpy. It's much worse."

"How could it be worse?"

"The crack was forced open. Normally a forced open crack will close once the machine doing the forcing has left. But the TARDIS isn't an ordinary machine. It's the most powerful machine in the cosmos, probably the multiverse. The hole it left open isn't going to close. In fact it's getting bigger, and it's probably going to keep sucking in more and more things."

Derpy was silent for a moment before replying, "So...what you're saying is..."

"If we can't find a way to close that hole, Equestria is going to become overrun with humans very soon."