Old Doctors never die, they simply fade away. That's exactly what had happened at the end of the Time Lord's epic first encounter with the deadly Cybermen.
Announcing that his old body had worn out, William Hartnell's Doctor collapsed on the floor of the TARDIS and transformed into the younger form of respected character actor Patrick Troughton.
The concept of 'Regeneration' was a brave one for the production team, with Troughton himself unsure it was a smart move. He would later tell Fantasy Empire magazine, 'I didn't think it was a particularly good idea of the BBC to replace Billy. I thought it was pretty silly really. I didn't see how anyone could follow him. In the end, I was persuaded over a week of negotiations, and I thought 'what the heck, let's do this for a while and see what happens'.'
Several ideas for the character and look of the new Doctor were bandied about before the idea of a Charlie-Chaplineque 'cosmic hobo' was settled upon. The Second Doctor's costume was something of an exaggerated version of his predecessor's - note how it seems to regenerate with him.
Early accessories such as the stove pipe hat were soon dispensed with and the extremely baggy (at Troughton's insistance) trousers were gradually taken in by the costume designer each week - without the actor's knowledge - until they looked a little more respectable. The trademark recorder the Doctor tended to tootle on during times of stress was the actor's own.
In a smart move, the new Doctor's arrival was ushered in with the return of an old enemy. The perennially popular Daleks blasted back onto TV screens in the first adventure penned by someone other than their creator, Terry Nation. Original series script editor David Whitaker took over instead and did a fine job of writing for the metal meanies. Scenes like the Dalek production line are now considered classic parts of Who mythology.
Sadly, no episodes of this landmark story remain in the BBC archives, although a couple of clips - such as the Dalek production line - still exist.