Doctor Who: Why Each Doctor Was Forced To Regenerate

13. The First Doctor (William Hartnell)

This is it - the regeneration that started it all.

The First Doctor met his end in the four-episode serial The Tenth Planet, which first aired all the way back in 1966. After the threat of the Cybermen has been nullified, the Doctor wanders back to his TARDIS, alone, with his constant battles and adventuring (and his great age) having taken their toll on his rather frail body.

The next time we see him, he falls on the floor of the console room and his face starts to glow: the first regeneration in the history of the show has begun.

Decades later, the 2017 Christmas special Twice Upon A Time gave us some more details about the First Doctor's regeneration, revealing that he squeezed in a final adventure with the Twelfth Doctor right before returning to the TARDIS.

Twice Upon A Time also revealed that the First Doctor didn't want to regenerate, a problem that the Twelfth Doctor is also facing in this episode. But together, they overcome this feeling and accept that it's time for them both to move on.