This idea. This idea. I... I just had to.
It all started when the eleventh Doctor was walking down the hallways of the TARDIS. He did that a lot, but this time was different. This time, he was walking down the TARDIS hallway since he had heard a strange noise. It had sounded like someone had fallen into the TARDIS.
He hadn't heard this sound since Clara had decided to go parachuting inside. Long story short, it didn't work very well. And the reason the Doctor was so suspicious was that no one else was on the TARDIS. Clara was off visiting her grandparents, and River normally gave him a message if she was going to come and see him.
The Doctor lit up his sonic screwdriver and edged into the room. He gasped when he saw the person inside. It was a very familiar face, the face of a man he never wanted to forget. It was his Tenth incarnation. The Tenth Doctor looked up at the Eleventh. "Hello. I'm the Doctor."
The Eleventh Doctor fingered his screwdriver awkwardly. "I know that."
"How'd you know that? I've never met you."
"Nope, but I've met you."
"Not again... River was bad enough…"
"It's not like that!"
"Then who the hell are you?"
"I'm the Doctor."
"Which number?"
Eleven started counting on his fingers through the faces. "Sorry about this… I get mixed up sometimes, but I'm pretty sure I'm eleven."
"Ten, but I suppose you know that?"
"Yes, and anyway, what happened right before you got here?" Eleven knew that he couldn't tell Ten anything he didn't already know. But he was aware that this was probably some kind of time paradox since he couldn't remember this from his past.
"I was visiting Rose Tyler in 2005. I was about to regenerate."
Eleven nodded. He knew exactly where Ten was in his time stream. He should have been regenerating, but he wasn't. Why was that? Eleven couldn't seem to figure it out. But the time for a friendly get together was over. A large box was staring them in the face, one that had just appeared.