Landmarks No.7

The Tenth Planet.
Significant for two very good reasons.
One is the introduction of the Cybermen - the second most popular monsters in the history of the programme - and the other is the first regeneration.
The Cybermen were devised by Kit Pedler, who had been brought onto the programme by story editor Gerry Davis as a sort of unofficial scientific adviser (after astronomer Patrick Moore had turned down the role). Pedler was concerned with the advances in transplant surgery. What would happen if you replaced everything that made you who you are? As well as replacing limbs and organs, what if doctors tampered with how we think and feel.
The Cybermen were created as emotionless cyborgs, a necessary step in order to survive the wastes of deep space after the planet Mondas (Earth's long-lost sister world) broke free of the Solar System thousands of years ago.
The original costumes were a bit rough and ready (with bulky chest units and helmets) and human hands were retained to show these were not mere robots. At the end of the story, the Cybermen are totally wiped out - even their planet disintegrating.
The producers of the programme had long been searching for another monster to rival the Daleks, and had been unsuccessful with the likes of Zarbi, Mechanoids and Monoids.
The Cybermen finally offered the potential of a foe that could be brought back again and again. The destruction of Mondas could be easily overlooked - they having another base on another world. (In their next appearance -The Moonbase - the question of where they come from isn't even referred to, and it isn't until their third outing that we learn of Telos).
It's fortunate that the Cybermen did prove so popular, as Terry Nation was about to withhold the Daleks from the programme to launch them in their own US series.
The first regeneration was never described as such. In Episode 1 of The Power of the Daleks, the Doctor says he has been "renewed", and the process is part of the TARDIS. Strangely, his clothes renew with his body.
Hartnell had almost been replaced during The Celestial Toymaker - turned mute and invisible, he would have been brought back played by a different actor. This was mainly due to the breakdown in the working relationship between Hartnell and producer John Wiles. Hartnell was also beginning to show signs of the illness that would eventually take his life, and was struggling with his workload.
Wiles left, and the new producer - Innes Lloyd - planned a means of replacing the Doctor with a new actor - not in a Miss Ellie / Dallas or a James Bond sort of way, but as part of his natural lifespan, he being an alien after all. It is not totally clear why the Doctor regenerates. It may be due to the Doctor simply reaching the end of his first incarnation, or Mondas' energy draining influence may also play a part.
Whatever, the TARDIS certainly plays a part, and it is significant that most subsequent regenerations take place in the ship. Indeed, the Tenth's regeneration into the Eleventh almost tears the ship apart.
There is little doubt that we would not be about to celebrate the 50th Anniversary of an active TV show if the production team in 1966 had not thought up the concept of regeneration.