Caught mid-regeneration after she was forced to regenerate by the Master the Doctor found herself trapped in her own consciousness. Here, she met the Guardians of the Edge, a single person with multiple faces.
Sound familiar?
Each of these faces was a previous incarnation of the Doctor, specifically the First, Fifth, Sixth, Seventh and Eighth Doctors. Whilst in her regenerative coma-state, the Doctor was able to reach out to her friends Yaz, Ace and Tegan, who were fighting Cybermen,the Daleks and the Master. How? As an interactive AI holo-Doctor, built from data of their behaviour across thousands of years. When they did this and met Ace and Tegan, their appearance switched to the Seventh and Fifth Doctors respectively, as their companions remembered their Doctors. The holo-Doctor even donned the appearance of the Fugitive Doctor to trick the Master, who had no idea who she was.
And of course, this latest episode saw the regeneration of the Thirteenth Doctor, into the Fourteenth Doctor. Eight Doctors in total!
Happy Doctor Who Day!