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Despite regeneration being a crucial part of the franchise's mythology, Doctor Who almost undercut its first regeneration. The concept of regeneration is the key to Doctor Who's longevity, for it allows the BBC TV show to change its lead actor whenever one wishes to move on. Regeneration stories often serve as an opportunity for Doctor Who to reinvent itself, and in recent years there have often been changes in showrunner at the same time a Doctor left.
The first regeneration - when William Hartnell changed into Patrick Troughton - occurred in 1966's season 4 story "The Tenth Planet." Hartnell's health was deteriorating; he suffered from undiagnosed arteriosclerosis, and its impact was most notable in affecting his ability to learn his lines. Matters weren't helped by a change in the production teams, and Hartnell had increasingly found himself clashing with them behind-the-scenes.
Hartnell actually came close to leaving some months earlier, in the Doctor Who season 3 story "The Celestial Toymaker." His contract was due to expire, and producer John Wiles - frustrated at their adversarial relationship - hit upon a plan to write Hartnell out of the show. This story villain was the Toymaker, who could rewrite reality at will, and for much of the story the Doctor was invisible and even mute. Wiles intended the story to end with the Toymaker making the Doctor visible once again, but revealing he now had a completely different face. Wiles' bosses overruled him, however, and Hartnell stayed on for a little while longer. When he did leave, a very different mechanism was used to write him out - one that assured Doctor Who's longevity, for unlike the approach proposed for "The Celestial Toymaker," it could be repeated.
What Would've Happened In Doctor Who If John Wiles Followed His Hartnell Plan
It's easy to forget the Doctor's story was only explored over the decades. The 1969's Doctor Who "The War Games," although it would be another five years before their portrayal was really locked down.
Doctor Who's mythology is, therefore, accretionary in nature. It's fascinating to imagine what would have happened had Wiles had his way in 1966, with Hartnell leaving in a reality-rewrite rather than a regeneration. The TV series would surely have gone in a very different direction, but the decision to make the change in lead a one-off event would have made it difficult for Doctor Who to repeat it. Regeneration allowed Doctor Who to last 60 years - and Wiles' idea wouldn't have done the same.