Doctor Who: EXACTLY Where Jo Martin's Doctor Fits Into Doctor Who Canon

The easy answer is that this is a Doctor who existed before William Hartnell, the First Doctor, appeared on screen in An Unearthly Child way back in 1963. After all, 1976's The Brain Of Morbius showed a mental battle during which past versions of the Doctor were shown, including several we haven't seen on screen before.

So the First Doctor wasn't actually the First Doctor, right? Perhaps, but there are a couple of things that don't fit with this theory.

The first is that Martin's TARDIS is already in the form of a police box. Introduced into the UK in the 1920s and operating until the early 1970s, police boxes were part cell and part telephone for officers and the public to make emergency calls in the days before mobile phones and radios.

When the first Doctor landed in 1963, his TARDIS changed form to that of a police box as they were everywhere and its chameleon circuit promptly got stuck. It was while he was trying to fix the TARDIS that his granddaughter's knowledge of the future led her teachers to investigate, making them the Doctor's first companions when he kidnapped them to escape detection.

This first adventure also contains the moment when, considering killing a caveman, the Doctor makes the vow to himself to never be cruel or cowardly. This is when he truly names himself The Doctor and defines what that must mean to himself.

As our newcomer already has that name and a police box TARDIS, the Hartnell Doctor must be somewhere in her past. But we've seen most of the regenerations that have taken place since Hartnell became Patrick Troughton, watching as one Doctor fell and another rose in his place.

Most, but not all.