Work Text:
The extra TARDIS (the TARDIS of the Doctor she barely remembered being, but she wouldn't think about that just yet) folded in on itself and the Daleks, sending all internal and external dimensions into the heart of the Void.
"Did we do it?" Jack said as he, Ryan, and Graham materialized in her own TARDIS, the proper TARDIS. The Doctor opened her mouth to reassure him that they had done it, Daleks defeated, but suddenly her timesense ground her outside perception to a halt.
There were only a very few situations where a small action on her part would result in completely different threads of time moving forward. The last time had been in that barn, hand (hands) hovering over the big red button that would destroy everything she was supposed to love. Not even that second destruction of Gallifrey had given her so much control over the way the universe would be remade, because she had already seen the destruction that would be wrought.
She had been careless in her programming of that other TARDIS. No, worse, she had been unkind. And now she would have to face one (1) universe of consequences.
The Doctor zeroed in on the three threads. They were so different, and that was the only reason she was able to see her own folly so clearly. Follow the threads, look at what the immediate and not-so-immediate consequences would be if she chose. When had she stopped thinking about the threads and the consequences.
One, two, three threads. One, two, three universes, all that would require her input soon lest they collide and create rifts and pocket impossibilities.
Universe Number One: The most likely universe, the one that made sense. A universe that she could not allow. Because the TARDIS she had told to fold in on itself, to destroy itself, was hers.
How had she forgotten that? Even if she was barely beginning to cope with the revelation of the past she had never known, how had she forgotten that the TARDIS had been the one constant through all of it? She had stabilized the Doctor when the CIA had wiped her memory again and again. She had held her, cradled her. She still did.
It was a paradox, because the Doctor would not have ordered that past version of her TARDIS on a kamikaze mission if she had not been in this circumstance. The destruction relied on the TARDIS continuing on, and if the Doctor didn’t tear through the Fabric of Reality and force the universe into a different shape… well, nothing was stable. Everything was in flux.
If she didn’t tear through everything in the right way, her present TARDIS would disappear. They had landed on solid ground, so it wasn’t as if she would be stranded in deep space, but it would still strand the Doctor on Earth.
That universe was unacceptable.
So, Universe Number Two: the Doctor could sever her TARDIS’s past from its present. She would still be able to escape into the Vortex; everything would be more or less as before. The TARDIS would hold no memory of a time before its own, pre-programmed destruction, but that was of little importance. The problem was Yaz.
If the Doctor severed her TARDIS’s past from its present, she would also be severing the connection Yaz had made with that ancient, young version of the TARDIS. The telepathic tendrils ran deep, nature and technology having crafted that race into the perfect symbiotes. If the Doctor saved her ship by rending its timeline, she would be rending Yaz’s mind along with it. Less unacceptable than being stranded in one place and time, but only marginally. She wouldn’t choose that universe if she could help it.
The only option left was Universe Number Three, and that was the most frightening universe of all.
She’d been hiding from the TARDIS those long months in prison. The isolation of being alone in a mind that was meant to carry others had nothing on the terror of her TARDIS knowing everything. Every memory, every mistake, every mind wipe would be laid bare for both of them. At least when she was alone she could close those doors, cordon off her newly-perceived timelines for a while. The TARDIS didn’t know and had no need for that mercy.
If she hadn’t still been closing those doors, there was no universe in which she would have forgotten that both the TARDISes were one and the same. With a bootstrap and a grandfather paradox, she would be able to wipe her mistake away and meld the two realities, the Schrodinger’s universes of destruction and life, into one where her TARDIS was alone but alive.
All it would cost her was everything.
Her TARDIS - She knew her TARDIS. She had forgiven the Doctor so many mistakes, but there were threads that could not be forgiven. Destroying her, through negligence and carelessness, was most certainly one of them, and if the Doctor asked her for help the TARDIS would see exactly how little she had valued her oldest friend. Everybody lives, all minds remain intact, but it would be a long time before the Doctor would be able to travel without feeling cold metal where there should be life. It would take lifetimes for the TARDIS to forgive her, if she ever did.
But compared to Universe One and Two… It wasn’t a choice at all. Or, rather, it was a choice, and one where the Doctor had to choose. There was only one way to choose and not destroy herself completely.
The Doctor opened her mind, the TARDIS came in, and the roots that nurtured them both shuddered with the poison that she had carelessly put in the water.