There was once a man…a crazy, brilliant man with shaggy hair and small eyes.
If you caught him in the right light his eyebrows disappeared or his eyes widened, adding depth to his foreign features. He wore a jacket of tweed with awkward elbow patches, a light-colored button down, slacks, boots; more importantly he sported a bow tie because 'bow ties were cool'. Some would call him fashionable while others would call him naïve disillusioned and just a bit off, those who knew the man were also knew that he was none of those things.
His name was the Doctor, just the Doctor, and he knew exactly what he was doing.
When it came to rules, he had only a few: Do exactly as I say, I'm not a professor, don't wander off, Time is not the boss of you. But the most important rule that he never spoke of out of practice was the one rule he held up as often as possible, the one impossible rule that could never be broken unless the universe absolutely had to break it – and it had, time and again. The Limitation Effect has been said to be the only standing effect in the universe and it told of meeting oneself, causing time to sort out the time differential between the two iterations of the person and causing a complete alternate timeline for the being. This was the one effect the Doctor would not cause if the universe didn't make it so, the one unbreakable rule.
The Doctor never crosses his own timeline.
Rule number one.
The Doctor always lies.
No one knew of the effects that were to pass in the universe when two of the Doctors collided, no one knew of the past visits the Doctor had made to himself merely because it was his time to do so. More importantly; no one knew that when the Doctor regenerated he left an empty shell of his earlier face on earth, leaving it to the universe to give the shell a life, a name and a place in existence once the Doctor continued on. Once the body was in its dying stages the shell would be left without any prior memories of the Doctor, the TARDIS, and the companions but would have a mind filled with false memories of a life that had yet to have been had. A completely different person that no one questions, no one suspects and everyone in their lives seems to remember as if they'd been at the hospital during their birth.
Shells are always created for Time Lords, springing up on humanoid planets seeming spun to perfection to fit into the race there. One heart, proper amount of stomachs and kidneys, maybe a scratch or two from the imaginary coma the universe seems to set them in before their awakening. But what happens when a shell remembers? I'd asked the Doctor once, the Doctor only grew angry with the question and told me to fetch a few blankets because the temperature was about to drop inside of the TARDIS. So I left the question open if he'd wanted to answer it later, when he was ready.
This isn't the story of the Doctor answering my question or sending me off to answer it for myself. It isn't the story of the man and his companion that take on an army of shells. It also isn't a story that ends well, a story that's filled with happiness and laughter – no.
This is the story of the man who sacrificed every moral he had to keep the universe safe from itself and the lengths he'd go to just to be sure of it. This is the story of the last time lord in a constant race against the ticking hands of time; for he knows that his own time might come all too soon. The story that you're reading is the story about the day a shell remembered the Doctor and he broke the one rule he'd avoided breaking at all costs – he crossed his own timeline, knowingly, to keep the universe safe from itself.