What is bi-generation in ‘Doctor Who?’ The Doctor’s latest regeneration, explained

Warning: This article contains spoilers for Doctor Who: ‘The Giggle.”

60 years in, you’d think Doctor Who fans had seen it all by now, but the finale of the show’s three 60th anniversary specials just tore up the rule book in its much-anticipated introduction of Ncuti Gatwa’s Fifteenth Doctor.

Well, this one was always going to be a weird one. 2022’s “The Power of the Doctor” already made an unprecedented move by having Jodie Whittaker’s Thirteenth Doctor regenerate into… not the previously announced Gatwa but back into David Tennant. Now playing the Fourteenth Doctor instead of the Tenth, Tennant was the lead of these three specials, but the time came to belatedly hand over the sonic screwdriver to Gatwa in special 3, “The Giggle.” And yet it didn’t go the way we expected.

Yes, Doctor Who genuinely just did something we have never seen in six full decades of time travel. Let’s explain…

How does bi-generation work and how is David Tennant still the Doctor?

As excited as we all were for Gatwa’s arrival, Doctor Who fans were dreading having to witness Tennant’s Doctor dying again — after all, we’ve experienced that once before back in 2010 and it wasn’t fun the first time around. Sure enough, during his battle with Neil Patrick Harris’ Toymaker, the Doctor gets shot with a fatal blast from U.N.I.T.’s galvanic beam superweapon. He says his goodbyes and is engulfed in regenerative energy. We all know what happens next, right?

Except, no, we don’t. Once the energy has faded, the Fourteenth Doctor is still there. “It feels different this time,” he says, confused, and asks his two friends, Donna and Mel, to pull on his arms. They do and out of the Doctor’s form emerges… another Doctor! Ncuti Gatwa’s Doctor, to be precise. “You’re me?” the incredulous Fourteen exclaims. “No, I’m me,” responds Fifteen. “I think I’m really, really me.”

The impossible thing we’ve just witnessed, we’re told, is a bi-generation, “a myth” of the Time Lords that says that, very rarely, a Gallifreyan regeneration can cause the Time Lord’s next body to sprout out of the old one instead of replacing it. Neither is a fake, a duplicate, or a diluted version of the character. Both Tennant and Gatwa are the Doctor.

With the Fifteenth Doctor heading off to explore the universe, as per uze, Fourteen — at Donna’s insistence — finally allows himself to stay in one place for once, settling down in present-day London and joining Donna’s extended family. Except, he’s also got a TARDIS of his own, thanks to the Toymaker warping reality, and it just so happens that present-day London is basically the setting of every other episode of this show.

We have Ncuti Gatwa’s shining new era ahead of us, and that’s the main thing, but thanks to the wonders of bi-generation, David Tennant can now return to the Whoniverse anytime he likes. Do we smell a spinoff, Disney Plus?