At the end of The Tenth Planet, the Doctor collapses onto the floor of the TARDIS from exhaustion. Current companions Ben and Polly watch on as an alien glow covers him, the roar of the TARDIS starts up, and the white light eventually fades to reveal a stranger in the Doctor’ place. But it’s far from being over. This change, originally referred to as Renewal, and now known as Regeneration, is one of the key contributors to the programme’s longevity. While it
wasn’t uncommon for television actors to be replaced, here the entire character of the Doctor undergoes regeneration. It is an ingenious device that helps new viewers in becoming emotionally connected to their Doctor. The Doctor isn't gone he's simply a new version, one sharing the basic tenets of the old character, with a few idiosyncrasies thrown in. As the Tenth Doctor puts it to a bewildered Harriet Jones in
The Christmas Invasion (2005) ‘I'm him. I'm literally him. Same man, new face... well, new everything’.  Regeneration is a means to continuing the character but allowing him to evolve and change and, in doing so, ensure that he remains relevant.