Author's note: Many thanks to Primsong and Illyushadarling for encouraging me to post this here!

"But, Doctor, this is your last life. You can't get yourself killed now!"

The thirteenth Doctor smiled down at his companion - his best friend, feisty as ever - Sarah Jane Smith. "Who says it's my last life?"

"But..." Sarah fumbled for words. "But you told me yourself. Twelve regenerations, thirteen lives. Teeth and Curls was number four, Rooster-hair with the sneakers was number ten, and you've regenerated three times since him. Thirteen lives."

The face might change, but that grin never did. "Ah, but that's not quite accurate. I referred to each of my different appearances as a different regeneration, for convenience's sake, but I do actually have one more coming."

"You mean you once changed your appearance without regenerating?" Sarah silently ran through the changes of face she knew about and the reason for each one. They'd all been fatal.

"Exactly. Well, not exactly. When the Time Lords captured me during my second life, they didn't actually execute me before exiling me. I suppose they felt it was too barbaric."

"Then... But I've seen pictures of you. Him. The second you. A little black-haired fellow with a face like a basset puppy, right? And the way he dressed – he was the exact opposite of my first Doctor."

"Yes. In every way, the exact opposite."

The Doctor obviously thought that was tremendously significant, but Sarah couldn't figure out why.

"So what did they do to you, if they didn't... regenerate you?" she demanded.

"Well, actually..." The Doctor rubbed the back of his neck sheepishly. "To be honest, they – well…"

"They what?" Sarah prompted.

"They reversed my polarity."