Chibnall has said that this is “definitively” the Doctor. That’s how she
was credited at the end of the episode. He’s also categorically denied that
this is a Doctor from a parallel universe. That leaves, I think, four valid
possibilities, three of which require a memory wipe:
A Doctor from an alternative timeline. This needs the least reworking of
the mythos: every Doctor after the first could be different in such a
timeline.
A pre Hartnell Doctor (the previous cycle of regenerations, as per the
Cartmel masterplan’s “Other” but without the name change).
A Doctor between Troughton and Pertwee, a sort of extension of the old
“season 6b” theory. Note: this need NOT violate the regeneration limit -
the Time Lords sentenced the Second Doctor to “a change of appearance”, not
a regeneration. So if either the change to the Jo Doctor or the change to
Pertwee was “a change of appearance” rather than a regeneration, only one
would be used.
A future Doctor. Personally I’d think this was the most fun option, and
it’s probably the simplest for a modern audience to understand, but then
I’m not Chris Chibnall.
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