Timeless Child," a being from beyond the universe who became the base genetic code for the upper class of Gallifrey.
Meanwhile, earlier episodes introduced another wrinkle; a mysterious, never-before-seen not the 'new' Doctor.
But there's no reason to assume Martin has been told all Chibnall's plans for her Doctor; and meanwhile, there is strong evidence to indicate this character may actually be a future incarnation of the Doctor.
Ruth Doesn't Easily Fit Into The Pre-Whittaker Doctor Timeline
When viewers were first introduced to Jo Martin's character, they thought she was simply a human being, mysteriously hunted by the Judoon. Little by little, though, the lies were stripped away, and the truth was revealed; she was actually the Doctor, a mystery incarnation who had hidden herself away on Earth in order to escape the Time Lords. She'd used the Chameleon Arch, technology previously seen in the David Tennant era, and had buried the TARDIS in the ground. Whittaker's Doctor swiftly deduced Martin's was a past version, because Gallifrey had been destroyed in the present, and because Martin's Doctor didn't recognize the sonic screwdriver.
There is, however, one striking problem with this theory; Martin's Doctor is piloting the same TARDIS, which is already stuck in the shape of a Police Box. This contradicts all the established canon of Doctor Who, because according to the episode "The Doctor's Wife," the Doctor has no prior history with the TARDIS before the fateful day he stole her. Meanwhile, the Chameleon Circuit only broke when the TARDIS landed on Earth in 1963, with William Hartnell's Doctor settling down so his granddaughter Susan could have an education. That's made clear in the very first Doctor Who story, "An Unearthly Child," in which Hartnell looks at the TARDIS' now-stuck exterior and complains about the Chameleon Circuit having broken down. Other Doctors have frequently referenced this.
The Doctor Who season 12 finale confirmed there are indeed additional past incarnations of the Doctor, their existence previously concealed by a mind-wipe. But Chibnall deliberately avoided confirming whether or not Ruth is one of those "new" past Doctors. "Where do you fit into all this," Whittaker's Doctor challenged when she confronted Martin's in the Matrix. "Were you me all that time ago? Were all my memories of you erased?" No answers were forthcoming, suggesting the mystery should still be considered unresolved.
Ruth Being From The Future Explains Why The Mystery Is Unresolved
The Judoon add another wrinkle to the plot, because something about their story just doesn't make sense. These brutal aliens fancy themselves as the galaxy's policemen, and in Martin's era they are working for the Time Lords. That seems distinctly odd, given during the Tennant era they were part of the Shadow Proclamation. In the episode "The Stolen Earth," representatives of the Shadow Proclamation revealed they had believed Time Lords to be nothing but a legend, a cosmic myth. It really doesn't fit with the idea the Judoon had used to work for the Time Lords, because it's reasonable to suggest their knowledge of galactic affairs had been shared with their colleagues at the Shadow Proclamation.
At the end of "The Timeless Children," the Judoon burst into the TARDIS and arrest Whittaker's Doctor, fulfilling what they called a "cold case." But if the Doctor was a Judoon cold case, why didn't they arrest him before, during the Tennant era? He certainly identified himself to the Judoon in "The Stolen Earth," and they appeared to have intimate recollection of all cold cases due to their obsessive nature.
On close examination, the final scenes of "The Timeless Children" offers a hint all is not as it seems. The arresting officers who enters the TARDIS is the same one who attempted to capture Martin's Doctor on Earth in "Fugitive of the Judoon." There, Martin's Doctor brought shame to Pol-Kon-Don by removing her horn, a demonstration of her superiority. But the horn is back in "The Timeless Children," suggesting things are happening out of sequence. As far as the Judoon are concerned, their arrest of Whittaker's Doctor precedes their pursuit of Martin's. As the Doctor might put it, it's all a bit "timey-wimey."
What Ruth Being The 14th Doctor Means For Doctor Who
If this is the case, the Doctor has unwittingly caught a spoiler-filled glimpse of her future - and she has even met her future incarnation. It's true Martin's Doctor claimed to have no recollection of the encounter with past self - but that may not be significant. The Doctor has always tended when he or she feels it is necessary, and in any case past multi-Doctor stories have suggested memories become blurred and indistinct whenever different Doctors team up. It's entirely possible she was unable to anything until she parted ways with Whittaker, and then the memories snapped into place.
This suggests Chibnall has taken a bold choice, casting the new Doctor in secret before Whittaker has even left, and introducing her to viewers so people can get used to her. Meanwhile, he has also teased the Time Lords as a resurgent force in the cosmos, returning once again after their apparent destruction at the Time Lords of Gallifrey, because she now knows she was no Time Lord at all, but rather their lab rat. It's not hard to imagine a scenario where the Time Lords themselves become the Doctor's greatest foes - masters of time and space, no longer eschewing intervention but instead enforcing their will upon all reality.
If this theory is correct, Whittaker is moving towards her last season, and the pieces are being moved into place for the endgame. She has been arrested by the Judoon, and is certain to escape, thereby becoming their fugitive - and, by extension, fugitive of their Time Lord masters. But, if the Doctor figures it out, the encounter with her future self means she can consider herself forewarned; she knows the Time Lords are coming back, when it's likely they would prefer to act in the shadows right now.