Landmarks No.2

The Daleks.
Or The Mutants if you prefer.
Doctor Who's second story is a landmark for two obvious reasons - it is the first of the purely futuristic, sci-fi stories which would come to predominate, and it introduces the Daleks.
It was always intended that Doctor Who would be a science fiction programme first and foremost, with educational elements thrown into the mix mainly through the historical adventures and the planned "sideways" stories - where the ordinary and everyday would be given some sort of a twist. Sidney Newman famously ordered that there be no BEM's in the show (Bug-Eyed Monsters), and he thought that was exactly what he was getting with the Daleks. Not only were they far more complex than that, but they were also very, very popular.
The significance of the Daleks can never be under-estimated.
Their arrival so early in the series' history helped to cement its popularity and increase interest. Viewing figures leapt up as word got round about the bizarre machine creatures (An Unearthly Child had 4.4 million viewing, and the final part of this story had more than 10 million).
Would any other monster have made the same impact? I don't think so. I can't see the Zarbi or the Voord becoming a mainstay of the programme had they been the first monster to appear in the programme (though the honour of being the first monster seen on screen actually falls to the Magnedon).
The Daleks owe their success to the combination of design, voice and their back-story. These are not B-Movie robots, but armoured life support machines containing the mutated relics of a neutronic war. They are xenophobes, who want to destroy anyone who is not like them.
The voice could be imitated by school children up and down the land (with or without your hands stuck out rigidly in front of you and a cardboard box over your head).
Today, the Daleks are synonymous with the programme - being used to launch new series on several occasions (or season finales). JNT used them sparingly, having only one full adventure with each of the last three classic series Doctors.
They have cameo'd in many stories, and have a life outside the programme - making numerous TV appearances in everything from game shows through comedies to adverts.
The vast majority of Doctor Who merchandise is Dalek based (or so it certainly feels).
The period 1967 - 1971 proved that Doctor Who could survive without the Daleks (but the Daleks certainly could not survive without Doctor Who).
Watching The Daleks back today - in one sitting - you see the obvious padding of the later episodes, but at the time the splitting up of the time-travelers and the prolonged quest through jungle and caverns would have actually sustained interest.
Many elements from this adventure will be reused by writer Terry Nation in future stories.
This story also forms the basis for the first of the cinematic Dr Who adventures, starring Peter Cushing.