Regeneration has been a recurring element in Doctor Who, as every time the eponymous Time Lord is mortally injured, he becomes a new man with a new personality as his body heals.
The former had all its episodes intact, while the latter’s third episode was represented as a “tele-snap” reconstruction.
As seen with other Time Lords, males can even become females thanks to regeneration, and vice-versa.
As Ben and Polly got used to this status quo change, they joined the Second Doctor in battling his arch-enemies, the Daleks, on the planet Vulcan.
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The First Doctor Regenerates Animation! | Doctor Who