That was a purely spectacular episode, at times genuinely chilling and those final minutes were captivating.
I think the planet the Doctor lands on at the start of the episode is Mondas at the time of ‘The Tenth Planet’ (a serial I have never watched but I am most certainly planning on watching it now). This gives weight to the theory that the Doctor will meet his first incarnation in the Christmas episode, kind of tying it to this two-parter. Perhaps the Doctor is suppressing his regeneration throughout the episode in a way similar to Eleven’s final scene.