“Just how big is this thing?” asked Twilight.
“The TARDIS?” asked the Doctor. “Big enough.” He pulled a lever with his mouth, and then added “It's got a swimming pool.”
“An outhouse with a swimming pool?”asked Spike. He shuddered.
The Doctor turned and glared at the little dragon. “It's not an outhouse.”
“Right. Well. Um. Talking about outhouses, I kinda need to go...”
Twilight frowned. “You should have gone before we left!” she insisted.
“We were going to an outhouse!” insisted Spike.
The Doctor glared at him again.
“To what I thought was an outhouse,” clarified Spike. “You don't go before you go to an outhouse, that would just be silly!”
The Doctor rolled his eyes. “Through that door,” he said, pointing with one hoof, “second door on the right. Not the second door on the left, that's the swimming pool.”
“Right!” said Spike, hurrying in the indicated direction.
The Doctor turned back to the control panel and started fiddling with the levers and switches again.
Several moments passed in silence, then Twilight asked “Do you, um. Do you have any idea why a future you might try to kill me?”
The Doctor nodded. “Yes,” he said.
Twilight stared at his for along moment.
The Doctor sighed, and left the control panel alone for the moment. “Imagine that there's a train,” he said. “It's rushing down the tracks, out of control. Far too heavy to stop. Someone has tied ten ponies to the track in front of the train; you can see that the train will ride over them. Now, you can't stop the train, but there's a lever right next to you that will divert the train onto another track.” The Doctor held up a hoof to forestall Twilight's comment. “Tied to the other track is one pony. Assume that all eleven ponies are complete strangers to you – you don't know who they are what they do, or anything. Would you pull the lever?”
“Um,” said Twilight. “I, um, I would probably cast an intangibility spell on the train...”
“Let's assume you've lost your magic,” said the Doctor.
“Um. Well. Um. I... I guess I would.”
“That would involve killing the one pony,” pointed out the Doctor.
“Yes, but it saves the lives of the other ten,” replied Twilight. “That's... well, that's nine better.”
The Doctor nodded. “I quite agree,” he said. “And that's why future me might want to kill you; to try to save more lives.”
Twilight frowned as she thought about that. “But...if something I was going to do was going to lead to ten ponies dying, and I knew about it, then I wouldn't do it. How would killing me save anypony?”
“I haven't the faintest idea,” admitted the Doctor. He turned back to the TARDIS control column. “Let's go find out. Hah! Strong trace... yes! I've found him! He's not even trying to hide his tracks.”
“You can hide your tracks time travelling?” asked Twilight.
“Oh yes. Dozens of ways. Let's see... he's going further back in time... um... how old are you?”
* * *
vworrrrrrrrp... vworrrrrrrrrp...vworrrrrrrrrrp
The sound echoed through the streets of Canterlot. It wasn't loud, as such, but an alicorn's ears are sensitive indeed; and the TARDIS faded into existence mere metres from the outside wall of Canterlot Castle.
Princess Celestia's ears twitched.
She smiled at the trio of ponies standing in front of her. “Mr. Shimmer, Mrs. Glass,” she said, “My school would be most glad to accept your daughter Sunset as a student – assuming, of course, that she passes the standard entrance exam.”
“Oh, she will, your highness,” said Looking Glass, with all the pride that only a mother can show.
“And in light of your financial situation, I will be happy to reduce the usual fee,” added Celestia. “Please talk to Pencil Pusher to get the necessary paperwork. Now, if you will excuse me, an urgent matter has just come to my attention. Steel Shield!”
Steel Shield, captain of the Royal Guard, snapped to attention, his armour jingling. “Yes, your highness?”
“Mobilise the entire guard. Double patrols, medium alert. There may be something... strange... around.”
Steel Shield looked confused; Looking Glass looked surprised.
“Strange how, your highness?” asked Steel Shield.
“I don't know, but if it's dangerous, we'll have help. I just heard an old friend arriving.” Celestia smiled, sadly. “He has a bad habit of turning up just in time to help defeat some ancient evil, and then vanishing again.”
Captain Steel Shield looked nervous. “Very well, your highness,” he said, and marched out to prepare the Guard.
* * *
“Canterlot,” said Twilight, following the Doctor out of the TARDIS. “We're in the Royal Gardens. I... I know this place.”
“Yeah, we both practically grew up here,” added Spike, sitting on Twilight's back.
“Excellent,” said the Doctor. “Now the question is, the important question is, can you remember exactly where you were on this particular day and at this particular time?”
“I was in nappies,” objected Twilight. “And I don't even know what day this is, in any case. Huh, that elm looks a lot smaller than I remember... I don't remember that stinkwood, that's where Celestia's picnic table was...”
“Will be,” corrected the Doctor. “Where Celestia's picnic table will be.”
“Um. Right.”
“Does anypony else smell something burning?” asked Spike.
* * *
The little baby purple unicorn gurgled and laughed at the pretty colours.
She was safe. She knew she was safe, because she was with Big Brother. Big Brother was a white colt who could always chase away any monsters hiding under the bed.
And she was doubly safe, because Big Brother's horn was glowing, sustaining a large, pink shield bubble around them.
Nothing could get through Big Brother's shield bubble, so the flames on the other side were just pretty colours to laugh at.
The flames stopped for a moment, as the grey stallion outside lowered his flamethrower.
“So, very noble and all, Shining Armour, was it?” said the stallion. “But ultimately useless. Consider – your shield either lets air in, or is doesn't. If it doesn't, you'll asphyxiate in there. And if it does...”
The stallion raised his flamethrower again.
“...then the superheated air you're letting in will eventually cook both of you.”
The stallion pulled on the trigger, and there were more pretty colours.
Twilight giggled. “Big Bwuvver!” she said.
Then the stallion spun round, throwing his flame in the direction of two ponies and a dragon, who had been rushing towards him from behind.
* * *
When the stallion turned his flamethrower on them, Twilight Sparkle had only a moment to react. She wrapped her magic around herself and the Doctor, and teleported off to the side. Then she put up her own shield in front of the two of them.
As Twilight vanished underneath him, Spike hit the ground, rolled, and charged.
“Where's the little guy?” asked the Doctor, trying to put out a fire in his mane.
“He's a dragon,” said Twilight, conjuring up a small raincloud over the Doctor. “He's fireproof.”
Spike, meanwhile, had reached the future Doctor; he took a deep breath, and fired a small wisp of flame of his own. It didn't do much beyond causing the stallion to take a few steps back, away from Shining Armour's shield; but that was all Twilight needed. Her horn glowed, as she put up a shield bubble of her own... around the future Doctor. A cooling charm dealt with the superheated air around Shining Armour's shield.
“And do you have any idea why the future me is all grey and faded?” asked the Doctor.
“I think,” said Twilight grimly, “that we are going to have to have a word with Discord about this.”
She took a good look over the trapped stallion. Aside from the grey colouration, the flamethrower, and the faded brown of his mane that suggested dye, the stallion looked exactly as he had when he's appeared in her castle; he still had a strange device wrapped around his forelimb, with a small screen and keyboard.
Twilight glanced aside. Spike was peering into Shining Armour's shield. “Hey, Twily!” he said.
The baby purple unicorn hid away behind her Big Brother's forelegs.
“Who are you people?” asked Shining Armour, staring at them “You!” he added, pointing at Twilight. “Are you Nightmare Moon?”
“What?” asked Twilight, shocked. “No! What gave you that idea?”
“You're not Princess Celestia, and there's not that many alicorns,” said Shining Armour. “And you're purple. That's close enough to dark blue.”
“Will you lot SHUT UP!” yelled the discorded Doctor. “I came here to kill you, not listen to you whine!”
“And perhaps you would like to explain to us all,” asked Princess Celestia, gliding to a stop above the group, “exactly why you decided to do that?” She nodded at the other time travellers. “Twilight, Doctor. Good to see you two again.”
“I don't have to explain a thing to you,” snarled the discorded Doctor. He pointed at Twilight. “You know what you did to me, where you trapped me! But I am Paradox! I am invulnerable, I am unstoppable I am ageless!”
The Doctor's eyes widened. “You didn't,” he gasped.
“I did!” cried Paradox. “I fed all the energy of my future regenerations into my biological matrix! I can't be killed, I can't be wounded -”
“You can't learn,” pointed out the Doctor. “You can't grow. You cannot, ever, change.”
“I know!” said Paradox. “Isn't it brilliant?” He gave the Doctor a mad grin. “And the best of all is, you can't avoid becoming me!”
He reached out a hoof, tapped a few buttons on the device wrapped around his foreleg, and vanished.
There was a moment of silence. The Doctor was the first to break it.
“He's... he's gone,” he said.
“Where to?” asked Celestia.
“Future... past... I don't know.” The Doctor waved his hooves about. “That device he had – it's not really all that accurate. He probably can't get closer than a month or two to when he wants.”
Celestia turned to smile at Shining Armour. “Young colt,” she said, “the danger has passed. You can lower your shield.”
Shining's shield vanished, and he looked up at Celestia in awe.
“Might I ask your name?” she said.
“Oh, um, Shining Armour, your Highness.” Shining suddenly remembered to bow. “And this is my little sister. Twilight Sparkle.”
“Indeed?” Princess Celestia raised an eyebrow, and glanced for a moment at the purple alicorn at her side. “Then these next few decades promise to be quite interesting.”
“Princess?” asked Shining Armour.
“Don't worry about it,” said Celestia. “Did you notice you've got your cutie mark?”
Shining Armour turned to look at his own rear, and his eyes widened.