Dec 10, 2016
In this scene from An Adventure in Space and Time, the biographical film that explored Doctor Who's earliest years, the first Doctor William Hartnell, played by David Bradley, passes the role onto Patrick Troughton. The concept of regeneration was invented to keep the show alive, but Hartnell could have never forseen how immortal both the series and the character would become as a result. The power of the Doctor's legacy is demonstrated with a cameo by then-Doctor Matt Smith, pulling past and present together in a way that the Doctor would truly appreciate.
Update April 18, 2017:
It seems that David Bradley's portrayal of the first Doctor was a hit! It has been reported that he will reprise the role in the 2017 Doctor Who Christmas Special. He will join Peter Capaldi's 12th Doctor in his last adventure as they save their home planet of Gallifrey by moving it into another dimension. If this sounds familiar, it's because it will tie into the 50th Anniversary episode, The Day of the Doctor, that gave us our first look at Peter Capaldi in the role ... or ... his eyebrows in the role.
In the climax of that episode, all Doctors from William Hartnell to Matt Smith joined forces to complete the task, with Peter Capaldi's Doctor swooping in to lend a hand. It will be revealed that this magic moment happens just before his regeneration.
Here's lookin' at you, kid.