Bi-generation was an extremely rare variant of regeneration which the Doctor believed to be a myth until their fifteenth incarnation bi-generated from the Fourteenth Doctor. Instead of one incarnation directly changing into the next, as seen with the regeneration process, bi-generation instead causes the new incarnation to split from the previous, allowing both to exist simultaneously. (TV: The Giggle [+]Russell T Davies, Doctor Who 60th Anniversary Specials (BBC One and Disney+, 2023).)
History
While "playing" with the Toymaker, who could alter physical reality to his whims, the Fourteenth Doctor bi-generated after being gravely wounded by a galvanic beam fired at him by the Toymaker to initiate a regeneration. As a result of the bi-generation, he continued to exist as an incarnation distinct from the newly created Fifteenth Doctor, who was still from his personal future. The process was assisted by Donna Noble and Melanie Bush, who each grabbed and pulled the Doctor's arms to separate the two incarnations, who both retained all their memories, and caused the garments the Doctor had on when the process occurred to separate randomly between the two of them, leaving both partially clothed. The Fifteenth Doctor also benefitted from the time the Fourteenth Doctor would go on to spend on Earth with the Noble family, remarking they had done "rehab out of order". (TV: The Giggle [+]Russell T Davies, Doctor Who 60th Anniversary Specials (BBC One and Disney+, 2023).)
The Fifteenth Doctor later stated the process felt like he "tore his soul in half", and feared he wouldn't be able survive bi-generating again. (TV: The Devil's Chord [+]Doctor Who (BBC One and Disney+, 2024).)
Effects
Bi-generation internalised and built up regeneration energy that would normally be expelled. This would cause discomfort, forcing the one undergoing the process to need help being pulled in opposite directions to initiate the release; once the heads have separated, the previous incarnation and the new one momentarily shared the body like conjoined twins, with each of them having their leg and being able to pull the arm from the other's side out.
Whatever clothes worn by the one bi-generating was split between the two. In the case of the Fifteenth Doctor, he got the dress shirt, tie, underpants, socks and shoes of the Fourteenth Doctor. Unfamiliar with the process of bi-generation, the Toymaker speculated that he could repeat it by killing the Doctors again, anticipating "vast meadows of Doctors dying over and over again", but was challenged to a game by the two Doctors before he could do so. (TV: The Giggle [+]Russell T Davies, Doctor Who 60th Anniversary Specials (BBC One and Disney+, 2023).)
Bi-generation has the unique effect of allowing new incarnations to benefit from any experiences and knowledge their predecessors are yet to experience or learn themselves, as the Fifteenth Doctor was shown with a much improved mental state upon emerging from the bi-generation as a result of the Fourteenth Doctor deciding to retire, something suggested to him by the Fifteenth Doctor himself, due to this the bi-generation results in the next incarnation of a Time Lord to be regarded as older than their immediate predecessor, (TV: The Giggle [+]Russell T Davies, Doctor Who 60th Anniversary Specials (BBC One and Disney+, 2023).) even though the Fifteenth Doctor was seemingly the same age as the Fourteenth Doctor, he had been pulled from the end of his predecessor's life into the future as his next incarnation. (PROSE: The Giggle)
Behind the scenes
- The Fifteenth Doctor's remark that bi-generation is "supposed to be a myth" implies the phenomenon was made possible due to the Toymaker's presence. Indeed, the Fifteenth Doctor realises he can similarly duplicate the TARDIS thanks to the lingering effect of "the Toymaker's domain", also described as "a state of play"; the Fifteenth Doctor's TARDIS explanation notably leads directly from a discussion of bi-generation.
- In the in-vision commentary and ODWP episode for The Giggle, Russell T Davies revealed his personal theory that the bi-generation affected the Doctor's whole timeline, meaning that previous incarnations would continue to exist beyond their on-screen regenerations, mirroring the audio drama series The Nest Cottage Chronicles by Paul Magrs, which was originally intended to depict a retired Fourth Doctor after his regeneration.
- Incarnations dying of old age like the First and War Doctors wouldn't survive bi-generation. Though it's unknown if the Eleventh Doctor would still get his youth reset before splitting with the Twelfth.
- An article speculated that the Fourth Doctor who Leela reunited with in The Final Battle [+]Pete McTighe, The Collection (BBC Studios, YouTube, 2024). had bi-generated.[1]