Doctor Who's finale leaves a pretty big TARDIS-shaped plot hole

But one question that definitely remains unanswered concerns the mystery Doctor played by Jo Martin, who was introduced earlier in the series in episode five (Fugitive of the Judoon) and kickstarted a new mystery as to why the current Doctor (Jodie Whittaker) couldn't remember this past life.

Now we know some of the answers, with series finale The Timeless Children (in which Martin appeared more fleetingly than might have been expected) revealing that - as many had predicted - she is all but certainly an earlier version of the Doctor, predating William Hartnell, and that any memory of her existence had been wiped from the Doctor's mind, along with all the details of the Doctor's other earlier lives.

While we don't know everything about her yet, we definitely have a basic idea of how she fits into the Doctor's timeline - barring one point. You see question marks remain over Jo Martin's TARDIS, and specifically the iconic blue police box form that it takes.

Of course, the police box TARDIS is perhaps the most essential piece of Who iconography there is - and it has been the preferred form for every Doctor that we've seen from William Hartnell right through to Jodie Whittaker. But as fans of the show will know, the time machine only came to take this shape during Hartnell's era as a means of blending into 1960s London.

What's more, we actually saw the first Doctor steal his TARDIS in 2013 story The Name of the Doctor when it hadn't taken the police box form yet, and so the fact that Martin's TARDIS is identical to the one we know so well despite her existing well before Hartnell has left many Whovians perplexed.

So what's going on? Well, as is often the case when it comes to Doctor Who, there are a number of possible explanations as to why this earlier Doctor's TARDIS has the same look as the one Hartnell's Doctor opted for years later.

Probably the most popular theory we've seen - and arguably the most likely one - is that the Doctor's TARDIS had taken the police box form prior to the Hartnell era, and that even though the character's memory had been wiped, some part of him/her unconsciously remembered this and ended up preferring the same form as his previous, forgotten version. The Doctor's memory had changed, in other words, but his/her tastes had stayed the same, which may have influenced the telepathic TARDIS' camouflage choices.

A similar theory - suggested by Bleeding Cool - posits that the TARDIS itself had taken this form, attempting to jog the Doctor's memory and remind him/her of his/her past in a manner that might evade the Time Lords who were so desperately trying to keep the past secret.

And despite the apparent implications of the final episode, some fans are still clinging to an earlier theory - that Jo Martin's Doctor actually didn't come before Hartnell in the chronology, but actually falls between the second incarnation (Patrick Troughton) and third incarnation (Jon Pertwee) of the Time Lord.

Given what we learnt in The Timeless Children, this latter suggestion seems unlikely. But then again, if there's one thing thats undeniable about Doctor Who it's that you never really know for sure –and in a few years someone might retcon the whole thing anyway.

Doctor Who returns to BBC One for a festive special in late 2020/early 2021