10 Doctor Who "Plot Holes" That Really Aren't
4. Why Did The First Doctor’s Clothes Change During Regeneration?
The First Doctor’s regeneration is noteworthy for a number of reasons. But perhaps most intriguingly, it was (until very recently), the only case of the Doctor’s clothes regenerating along with their body.
Putting aside The Power of the Doctor's ending for a moment, how can we square this with established continuity, given that every other Doctor has kept their predecessor’s clothes?
From a behind-the-scenes perspective, the reasoning is clear. When William Hartnell bowed out, the rules of regeneration were still being written (in fact, the process wouldn’t be referred to as "regeneration" until the Third Doctor's swansong).
With a little imagination, we can bend this into an in-universe explanation too. The fact that this was the Doctor’s first regeneration (Timeless Child notwithstanding) points to it being something unique and one-off – perhaps it’s something that happens to every Time Lord when they regenerate for the first time.
This doesn’t explain why the same thing happened to the Thirteenth Doctor, but we're likely to get an explanation for that come the 60th anniversary specials in November!