Bi-generation
Bi-generation was a variant of regeneration which the Doctor believed to be a myth. Instead of one incarnation directly transforming into the next, bi-generation caused the new incarnation to split from the previous, allowing the two to become independent beings.
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. This resulted in him continuing to exist as an incarnation distinct from the newly created Fifteenth Doctor, 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 each Doctor, 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 that Time Lords do "rehab out of order."
The Fifteenth Doctor later caused the TARDIS to undergo a bi-generation of its own, using the fact that he had won a prize for defeating the Toymaker so that both Doctors had their own TARDISes. He was able to accomplish this as the Toymaker and his rules still lingered for a few moments after his defeat. (TV: The Giggle [+]Loading...["The Giggle (TV story)"])
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. The two Doctors needed additional help, then, to fully separate.
Whatever clothes worn by the one regenerating 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; giving the newer incarnation some modesty. (TV: The Giggle [+]Loading...["The Giggle (TV story)"])
Behind the scenes
- In the in-vision commentary for The Giggle, Russell T Davies suggested that the bi-generation affected the Doctor's whole timeline so that previous incarnations may have continued to exist beyond their on-screen regenerations, explaining the Doctors seen in Tales of the TARDIS.
- This mirrors the audio drama series The Nest Cottage Chronicles by Paul Magrs, which was intended to depict a retired Fourth Doctor after his regeneration.
