Who is the new Doctor and where does she come from?

TV As both Russell T Davies and Steven Moffat illustrated, the best way to get people talking about Doctor Who is to present the viewer with a huge mystery then leave them hanging. It's a version of the JJ Abrams mystery box, except with a pretty good idea that there's a plan which will result in something debouching towards an explanation.

The reception for the Chibnall era so far within the wider discourse once everyone became used to the idea of someone from Skelmanthorpe playing the Doctor has been pretty lukewarm.  But this year has offered the one-two punch of the return of the Master and now a whole new Doctor, the first who's also played a neurosurgeon on Holby City.

Since Sunday night, there's been a definite shift back to the Bad Wolf days.  We've all become Vroomfondel and Majikthise, the philosophers awaiting Deep Thought's ultimate answer in The Hitchhikers Guide to the Galaxy but with just a few weeks or months to wait rather than seven and half million years.  Many, many websites are however cleaning up in the prediction business.

Ahem.  Well, here my fan theory.  Having already posted this on the bird based social media account, it's probably worth immortalizing here (depending on how your definition of "worth" goes).  I should mention that this is based partially on versions of theories I've heard from a few people now (including @JayStringer).  So this is an embellishment.

Where did the Jo Martin incarnation come from?

The Doctor was loomed/born and happily living on Gallifrey, regenerating now and then when a body is wearing a bit thin.  But eventually the Doctor is called up for service to do a mission, possibly go back in time and decimate the Cybermen.

It goes badly, or at least so badly that the Time Lords want to cover it up.  Post-mission he or she is at the end of a regenerative cycle, or indeed not but the Time Lords force the issue by getting he or she there artificially and the Doctor is given and starts a whole new regenerative cycle ("!").

As part of the process, he's regenerated into a Time Tot and his memory is either wiped or suppressed.  So as far as he can tell and indeed anyone else, he's a fresh new Gallifreyan, his first life (which includes Jo Martin) completely forgotten.  A timeless child.

All of which could explain why the Doctor loses her memory or senses after each regeneration even though most other Time Lords take it in their stride.  It's a side effect of what was done to her - her mind and body has to recover its sense of self.

It's also why various interventions like the Watcher have been used along the way.  Extra support is often needed to aid the regenerative process and even then, something often goes wrong and a mental cascade ensues (The Twin Dilemma, Pudsey Cutaway).

Why does she seem so different?

As for why the Jo Martin Doctor seems a bit more ambiguous and chippy, it's because unlike Hartnell and Capaldi who's connection to humanity eventually domesticates them, this Doctor only spends her time around Time Lords.  You could almost say that she's more ruthless.  Which in her case is literally true because she's gone from being Ruth to Ruth-less.

But why her TARDIS already a police box?

Assuming this wasn't a glaring continuity error, the earlier Doctor could already have been to London in the 50s or 60s so the TARDIS already has that shape embedded in its memory. It's confiscated and reconditioned when The Doctor is rebooted. The Clara fragment on Gallifrey somehow knows that which is why she points him towards it.  Either that of the TARDIS's telepathic circuits subconsciously suggest it to her and she suggests it to the Doctor.

The box then eventually returns him to London via Quinnis so that it can inconspicuously turn back into a Police Box perhaps in an attempt to jog his memory - the telepathic connection indicate that there is something wrong with her thief.  It then remains a police box not because the chameleon circuit is broken but because the TARDIS keeps that shape in constant hope that the Doctor will eventually remember their earlier life together.

Why doesn't she remember all this come The Doctor's Wife?  Well, the TARDIS has been through a fair few traumas by then and so she's "forgotten" that he had a whole earlier history with the Doctor so as far as she can remember, Hartnell was the first.

But what about?

Yes, I know.  If the earlier Doctor's been travelling somewhat too, how come "our" Doctor hasn't bumped into them before?  Why isn't it The Eighteen Doctors?  Well how about, what if the earlier regenerative cycle is "time locked" like the Time War to stop that happening, but when the Master destroyed Gallifrey he unpicked this time lock, causing these these two Doctors to meet.