Spoilers – not up to date on the latest season of Dr Who? Don’t read this!
So – my fan theory about who the new/old Doctor is. This is my theory, which is by me, and I own it.
Hem! Hem! My theory, by BG Hilton brackets mister close brackets:
The Ruth/Doctor’s TARDIS uses an older control room design. She refers to the TARDIS as a ship and spells out the acronym TARDIS – both 1960s Who ways of talking about the TARDIS but not used so much later in the series. She does not recognise the sonic screwdriver. Her costume is mostly old-fashioned and neutral but with a bit of bright patterned blouse at the collar.
All of this tends to put her early in the series
In the episode Twice upon a Time, the 12th Doctor meets the 1st Doctor just before he regenerates. At the end of that episode, we see Hartnell regenerate into Troughton as expected. However, in the original version of that scene, Polly and Ben are clearly at the Doctor’s side, whereas they are no longer present in the new version. I submit that this is evidence that the 12th Doctor’s intervention has somehow caused the 1st Doctor’s timestream to split. Hartnell experiences a simultaneous additional regeneration. There are basically two separate Second Doctors in their own TARDISes. The timelines of Polly and Ben do not split, so they remain with Doctor 2a, while his doppelgänger Doctor 2b goes off and has some solo adventure.
With me so far?
In time, Doctor 2b regenerates into Dr 3b and then into Dr 4b. Which one of these is the Ruth Doctor? My money is on Dr 3b. Here’s how it goes: after 2a rebels, he is finally forcibly regenerated by the High Council. Around about the same time, Doctor 2b dies, but does not regenerate into Jon Pertwee. If you remember, Troughton was given a choice of some potential regeneration forms but rejected them. The Time Lords chose the Pertwee form for him, presumably because it was the most aristocratic, ‘Time Lordish’ form. Dr 2b takes a very different form — Doctor 3b, henceforth RuthDoctor.
During the Omega crisis shown in The Three Doctors, the existence of RuthDoctor becomes known to a Time Lord (the one with the weird silver Beatles cut), who discovers her while trying to track down the past incarnations of Doctor 3a. He does not pass this information onto the High Council. Instead, the RuthDoctor is recruited by a rogue faction of Time Lords under threat of being returned to Gallifrey to face charges — since RuthDoctor is unaware that trial and punishment have already taken place.
What happens to RuthDoctor next is unclear – we’ll probably find out over the course of the next season or two. But eventually she dies and regenerates into Doctor 4b. Here, we are on thinner ice with figuring out what happened. But I suspect that Doctor 4b is very much like a substantially older version of Doctor 4a. Unhappy at being used as a puppet by the Time Lords, instead of travelling the universe righting wrongs, this Doctor resolves to learn everything he possibly can about his former masters. He finds it necessary to reveal some of this information to his future incarnations – usually by means of cryptic messages (‘Bad Wolf’, Hybrid’, ‘Lone Cyberman’) but on one occasion finds it necessary to talk directly to Dr 11.
Though both Doctor a and Doctor b are basically the same person, the mere existence of these two separate timestreams places a great deal of stress on the natural order of the universe. This eventually comes to the attention of the White Guardian, who needs the Key of Time to undo the damage. Blaming the Doctor for the situation, he chooses Dr 4a to find the six segments of the Key.
The White Guardian’s repair of the universe causes Doctor 4b’s timeline to end — but he does not die. Instead, he becomes the Watcher. The Watcher, forearmed with knowledge of future events, waits until Doctor 4a’s death is approaching and meets his other self in person. After 4a’s death, the Watcher is reabsorbed into the Doctor’s main timeline, becoming Dr 5.
Subsequent Doctors retain only conscious memories of Drs 2a, 3a and 4a, not the Doctor b timeline – but somewhere inside, perhaps there’s a little recollection? When a Doctor references a past adventure and gets it just a little wrong…
So that’s my theory, which is by me and I own it. But somehow I doubt the actual answer will be quite as involved.