Warning: Spoiler ahead for Doctor Who season 15, episode 8, "The Reality War."Doctor Who introduced bi-generation in 2023's "The Giggle," with the surprise twist allowing David Tennant's Fourteenth Doctor to essentially split in two, spawning Ncuti Gatwa's Fifteenth while Fourteen remained completely autonomous. Fifteen's final episode has finally provided the best explanation yet for why it happened, and why Anita Dobson's Mrs. Flood also experienced the same phenomenon at the end of "The Interstellar Song Contest." ittedly, it is only an in-universe theory, but it makes perfect sense for a moment of such throwaway speculation.

Bi-generation allowed Gatwa to have a groundbreaking Doctor Who debut near the Gatwa left the show and was replaced by Billie Piper.

The Rani's Bi-Generation Created A Plot Hole After Doctor Who Season 14

Bi-generation was supposed to be incredibly rare

The Doctor had heard of bi-generation before it happened to him in "The Giggle," but only in the context of Time Lord mythology. It was believed to be something of a fairy tale, and not something that could actually happen to a member of the species. At the time, this felt like Russell. T Davies was making sure everyone knew that it would be an isolated incident. That's why it came as such a surprise that Mrs. Flood bi-generated to create another Rani (Archie Panjabi) in "The Interstellar Song Contest."

It didn't really make sense that the Rani would experience bi-generation so soon after it happened to the Doctor.

While "The Giggle" revealed that bi-generation wasn't just a myth, it also strongly alluded to the fact that it was incredibly rare, if not a one-time thing. So, it didn't really make sense that the Rani would experience bi-generation so soon after it happened to the Doctor. At the very least, there should have been a much, much longer period before it happened to another Time Lord. The Rani's bi-generation scene made the Doctor's far less special, and opened the door to it happening again soon, which didn't align with the Doctor's original appraisal in "The Giggle."

At first, I thought that maybe the Doctor's new ability to bi-generate came from either his proximity to the Toymaker or his recently changed origin story brought about by Davies has essentially retconned the Timeless Child, so I don't think either of my original theories really stand up to scrutiny. Thankfully, Fifteen dropped a pretty convincing thought about bi-generation in "The Reality War."

It's The Master's Fault That Bi-Generation Is Now A Common Thing In Doctor Who

The Master kickstarted a slow death for the Time Lords

Sacha Dhawan's Master committed genocide in 2020's "The Timeless Children," killing all the Time Lords but himself and Jodie Whittaker's Thirteenth Doctor. Davies has now undone the Master's scheme, but replaced it with a new one. Instead of all the Time Lords dying in the Master's "genetic explosion," they were all made infertile, meaning there can now never be another naturally born Time Lord. It's still technically genocide, but a much longer, drawn-out way of bringing about the end of the species.

THE RANI: "We're sterile."

THE DOCTOR: "We are infertile."

THE RANI: "The species is barren. We're a biological dead end. There can never be another Gallifreyan child."

THE DOCTOR: "Sometimes, I think that's what bi-generation is. A life force trying anything it can to survive."

MEL: "What happened?"

THE RANI: "That genetic explosion. It didn't kill us, but it roasted us and sterilized us. Extinction is taking a bit longer, that's all."

- Doctor Who season 15, episode 8, "The Reality War."

The Time Lords' species-wide infertility is only revealed in "The Reality War," which leads Fifteen to voice his theory that bi-generation is an attempt to keep the species alive, with desperate Time Lord bodies trying to repopulate the species in new and creative ways. The Master's attack on the Time Lords happened all at once through time and space, but from a Time Lord's perspective, it's only recently happened. So, it sterilized the Thirteenth Doctor onward. Thirteen didn't bi-generate, but Fourteen did shortly after Whittaker's exit.

If there were more Time Lords still alive, it may be that more instances of bi-generation would have been reported recently.

The Rani also used her immense expertise in biochemistry to avoid the Master's Time Lord genocide, altering her own DNA, so the weapon failed to recognize her as a member of her original species. She presumably did this in her Mrs. Flood form, and then bi-generated when she became fatally wounded in "The Interstellar Song Contest." If there were more Time Lords still alive, it may be that more instances of bi-generation would have been reported recently. It remains to be seen how and if the Master's plan affected Susan, with Carole Ann Ford's character also returning in season 15.

Bi-Generation Becoming More Common Is A Problem For Doctor Who

The Fourteenth Doctor existing but never helping is enough of an issue

Ncuti Gatwa had another quirky regeneration scene when he stepped aside for the return of Billie Piper, but Fifteen didn't split into two like Fourteen and the Rani did. Gatwa's Doctor completely transformed in "The Reality War," avoiding similar loose ends to the ones created by Fourteen's continued existence. Although bi-generation is a cool twist on paper, it creates some pretty big problems for the show's future. Even if the Fourteenth Doctor is the only version of the Time Lord to ever bi-generate, it hurts his character a bit that he hasn't stepped in to help Fifteen since.

It would have been a much neater ending for Fourteen if he had undergone a traditional regeneration sequence.

Granted, Fifteen did just fine with saving the universe on several occasions, but I find it hard to believe that Fourteen would be okay with just sitting on his hands as aliens threatened Earth and those he loves. It would have been a much neater ending for Fourteen if he had undergone a traditional regeneration sequence rather than spawning Fifteen and continuing to exist as his own entity.

Tennant's Doctor fears regeneration far more than any other version of the Time Lord, so it seems fitting that he was able to come back and survive the process.

What's also unclear is Fourteen eventually becomes Tom Baker's Curator from 2013's "The Day of the Doctor."

Doctor Who season 16 still hasn't been confirmed, so Davies may be waiting a while before he gets to continue the story, if he is even given the chance at all. I really hope that at least some of the issues and questions raised by bi-generation are resolved in the show's immediate future. It wouldn't be the end of the world if they weren't, but it would be great to know that Davies has also been thinking more about his massive addition to Doctor Who's regeneration canon.