WARNING: The following contains major spoilers for the latest episode of Doctor Who, “The Timeless Children,” which premiered Sunday on BBC.
In the Season 12 episode "Fugitive of the Judoon," Doctor Who threw fans a massive new mystery with the introduction of Jo Martin's other Doctor. After her memories had been temporarily removed, this other Doctor lived life as a human on Earth. Eventually, however, it all came back to her, and she would reveal herself by showing Jodie Whittaker's Thirteenth Doctor her TARDIS. This new Doctor was, seemingly, the real deal, having the same code, the same attitude and the same colorful preference in clothes as the Doctor fans know and love. And yet, neither remembered the other, which meant they couldn't possibly be the same.
However, showrunner Chris Chibnall confirmed there were no tricks, claiming that Jo Martin's Doctor was truly the real deal. Fans were left with plenty of questions, and the answers have finally arrived in the Season 12 finale.
In "The Timeless Children," the Master delivers on his promise to the Doctor that everything was going to change going forward. After trapping the Doctor in the Matrix of the Time Lords, she learns the secret history of the Time Lords, and how they replicated their ability to regenerate from the Timeless Child, a young alien found at the bottom of a portal on a faraway planet a long, long time ago. The origins of the child are unknown, but she fell from the portal from, seemingly, another universe or dimension.
This child would become the basis of the Time Lords and, as is later revealed in the episode, the Doctor is this Timeless Child. Therefore, fans come to understand that the Doctor has had plenty of other lives prior to her first incarnation as William Hartnell. When she was a child, she went through multiple regenerations while her adoptive mother studied her ability -- and there is no telling how many others she went through before the start of the BBC series, 57 years ago.
In the Matrix, the Doctor eventually comes face-to-face with Jo Martin's Doctor, and it's there that she understands this version of herself actually predates her first incarnation as Hartnell. It's also at this point it becomes clear that the reason this other Doctor doesn't know the Thirteenth Doctor is that she is a later regeneration, with the answer as to why the Thirteenth Doctor doesn't know the other one being that her memory was erased under mysterious circumstances.
Of course, there is still a lot of questions surrounding the other Doctor, and why the Thirteenth Doctor's memories were erased by the Time Lords. However, now, at least fans seemingly know where Jo Martin fits in.
Doctor Who stars Jodie Whittaker as the 13th iteration of the titular Time Lord. Whittaker is joined by co-stars Bradley Walsh, Tosin Cole and Mandip Gill in a series spearheaded by showrunner Chris Chibnall. The next episode, "Revolution of the Daleks," will air this holiday season. The new season and all previous 11 seasons will stream on HBO Max later this year.