Show creator Russell T Davies revealed: “The real heart of it is the beginning.
“The Doctor arrives, hears a damsel in distress, he steps forward to save her when this other man swings in, dashing, brilliant, amazing, clever, witty, saves the day.
“The Doctor says: ‘Who are you?’ The man says, ‘I’m the Doctor!’ The Doctor becomes his companion. There will be a beautiful woman too, of course, but really it’s the Doctor paired with a new Doctor.”
David Morrissey plays Time Lord No 2 in the festive episode called The Next Doctor.
Russell added the next series of the hit BBC1 show would be sexier under new lead writer and executive producer Steven Moffat.
Russell, who is stepping aside, added: “Look at Steven — he’s a writer clearly driven by sex.”