When Chris Chibnall took over Doctor Who in Season 11, the series underwent something of a hard reboot, with a new Doctor, new companions, and a new attitude. But it also abandoned the "season-long arc," a hallmark of the series since its return to TV in 2006. With Season 12, the show returned to the multi-episode arc with a vengeance. The series premiere two-parter kicked off one, with the Master's mysterious destruction of Gallifrey over "The Timeless Child," and Captain Jack Harness arrived halfway through the season with the other to introduce the "Lone Cyberman."
But Doctor Who's Season 12 finale, in which both of these stories came together, revealed the plan was far more ambitious, as the arc's dénouement flipped the Doctor's origin story entirely backward. When the Doctor showed up on Earth in the rebooted Doctor Who Season 1, he was introduced to audiences as "The Last of the Time Lords." In "The Timeless Children," the backstory reverses it. She's now "The First of the Time Lords," the origin of her entire people. In short, the Time Lords' grand origin story was derived from the abuse of one small child.
In doing this, the show added a giant opening to fill in the blanks. It opens up the show both for Chibnall in Season 13 and for those who come after. With the companions also sent home, perhaps for good, the show has permitted itself to go anywhere next, and the series is better for it.
While certain sections of the Doctor Who fandom doubtless fell into hysterics, Chibnall's choice is derived in plot arcs from Classic Who. Most of the finale's twists were taken directly from Time Lord lore laid out in the original Doctor Who Series 14 episode "The Deadly Assassin" (1976). It also drew in an often-overlooked "continuity error" that occurred in Series 13's "The Brain of Morbius" (1976), where several faces of the Doctor appear that do not match any known regenerations.
But in Chibnall's hands, they become the basis of a Whoniversal expansion. The Doctor's established set of 12 Regenerations is merely one of many — perhaps thousands of — lifetimes lived, all redacted and forgotten, just waiting to be backfilled.
The sheer wealth of wide-open space this creates within the show's already established parameters is mind-boggling. Simply defining how far this expansion goes, and how many "sets of 12" there are of the Doctor's life, could take multiple seasons, let alone introduce a backlog of characters from the redacted parts. Does the show actually bring back those Morbius Doctors from the '70s, one by one? (There were eight shown altogether at the time.) Does next season group them to create a new "set of 12," one that includes Jo Martin's "RuthDoctor"? Consider what happens if those faces aren't all from a single previous set but Doctors come from different, randomized sets of 12. Each could be their own season-long arc in that case.
Filling in these new sets of 12 would be like a giant 3D galactic bingo cube. "Gotta catch 'em all" would suddenly apply to an entirely new franchise. Funko Pops! would have a field day.
But the expansion doesn't just open up the Doctor to brand-new stories and actors. It also tells fans a lot about the rest of the Time Lords. Previous showrunner Steve Moffat famously threw out the "12-regeneration rule" during his final years when the show reached the cap but needed to keep going. Chibnall has now reinstated it for everyone except the Doctor. That means the Master only has a few regenerations left, for example, as Sacha Dhawan marks the eighth incarnation of the character. Not that anyone is ready to see the Master go quietly into that good night yet, but it changes the game that he will eventually end, and the Doctor will keep going, lonelier than ever.
But more critically, Regenerating wasn't originally part of who these people were. The indigenous race to Gallifrey, the Shobogan, genetically engineered this into their ruling classes. That means Regeneration is something other races can also splice into their genetic code, given the right technology. Today the Master is combining Cybermen and Time Lords into some (genuinely nattily dressed) CyberMasters. Tomorrow, who could be picking up the ability? Can anyone imagine what would happen if Davros got his hands on this? Regenerating Daleks is almost as terrifying as the first time one finally figured out how to go up a flight of stairs.
Then there's this: The Doctor is not a Time Lord at all. She was not born Shobogan; her ability to produce that sweet Regeneration energy is all-natural. In short, she comes from a completely different alien race altogether. Who are the Doctor's real people? Where are they? The TARDIS has been to the ends of time and the deepest reaches of space. Have they been quietly there the whole time, watching her? Are they silently Regenerating in secret under the very noses of the egotistically self-styled Time Lords?
Season 12 ended on a cliffhanger. The Doctor is locked away for something that happened during her redacted years, and the companions are seemingly home on Earth for good. It's a sign that though fans got answers, this story has not ended. Chibnall has given us not just a season-long arc, but one that could consume the rest of his tenure if he were to let it. The Doctor's story will continue, while her past unfolds as well. Doctor Who may be 57 years old, but it's only just begun anew.