The Doctor and his crew - granddaughter Susan, and human schoolteachers Ian and Barbara - first met the Daleks on their second adventure. This story, as much as anything, solidified Doctor Who's place as television worth watching. The Dalek's striking appearance and shrill cry of "exterminate!" became a national phenomenon, sending millions of British kids to hide behind the couch. The only problem was that the Doctor essentially left the Daleks as he found them.
In that first series of episodes, the TARDIS landed on the desolate world of Skaro in the wake of the atomic war that caused the genesis of the Daleks. Along with the remaining band of Thals, who were in that disastrous war with the Dals, they invaded the Dalek city and destroyed them once and for all. Needless to say, finally destroying your greatest enemies once and for all creates problems for bringing them back by popular demand.
Writer and Dalek creator Terry Nation resolved this through an ingenious understanding of time travel. If someone is bouncing around through the fourth dimension, then it is entirely possible for them to have had their final conflict with their enemies before their enemies had their first contact with them. When Ian is shocked to see a living Dalek rise up from the Thames river, the Doctor chastises him, theorizing that their adventure on Skaro was millions of years in the future (I gather that this explanation was reworked and rendered false in later incarnations of Doctor Who, but we tend not to concern ourselves with those here).
Nation also recognized that there was only one thing he could solidly do with the Daleks on the basis of that demand: the Dalek invasion of Earth. If the Daleks hiding in their city on Skaro could send children behind the couch, then Daleks rolling through the streets of London would send them screaming in terror. That was slightly buffered by the Dalek's being a public spectacle on the days of location filming - the first major use in the production's history - where the evil machines were surrounded by adoring fans.
In The Dalek Invasion of Earth, the Doctor finally pilots the TARDIS back to Earth. To the chagrin of Ian and Barbara, however, the goal was overshot by two centuries. Instead of landing in 1964, they have come upon the devastated London of 2164. To further compound the problem, this England of two centuries hense has been conquered from space by the genocidal Daleks. Using their own might and an army of cybernetically enhanced and brainwashed humans called "robomen", the Daleks are capturing and shipping humans to Bedfordshire to haul rocks in their mine core reaching to the centre of the Earth.
Why the centre of the Earth? The answer to that was ingeniously fitted into current Doctor Who mythology in the series four finale The Stolen Earth and Journey's End. Nor was it the only reference. Lines of dialogue were transplanted outright, and during The Dalek Invasion of Earth, the Doctor triumphalistically declares that to conquer humanity the Daleks would have to destroy all living matter. A good idea, the Daleks think to themselves. It's very nice to see ancient Doctor history being brought into the modern version. Now if only David Tennant could be rendered in black-and-white for a CGI-enabled two-Doctor jaunt... However, back in the series at hand...
In fighting to rescue the planet from the plungers of the Daleks, the quartet falls in with the human resistance. In particular, Susan falls in love with the rebel leader David, remarking to him her longstanding feelings about how nice it would be to have some roots in a time and a place instead of charging around the chronosphere willy-nilly. Suffice it to say that she finds these roots with her human lover, leading to a rather touching farewell speech from William Hartnell to his departing cast member.
So this story was an ending. Carole Ann Ford, the actress behind Susan, left the show and the story itself was the last one filmed in the series' original production block. There was some question of whether or not it would be renewed. Once it was confirmed, The Dalek Invasion of Earth was shunted to the second season. Bounding the first and second seasons, the story also marks the essential transition between the original run of historical and scientific pedagogical stories and the outright Science Fiction that would dominate thereafter. There were only half as many proper historicals after this as there was Sci-Fi, in which the story developed out of the historical circumstance as opposed to having alien influences. It also wouldn't be long before the Doctor found his next young ward.
The Dalek Invasion of Earth outlasted the BBC video purges and is available on DVD. Included among the special features are an "enhanced" version of the story in which key effects shots of the Dalek ship flying over London have been changed out for new CGI shots replicating the type of Dalek ship more familiar to followers of the current Doctor.
The Dalek Invasion of Earth was also reinterpreted by BBC's Radio Times in their issue announcing the return of the malevolent pepper-pots in 2005. We have to admit to loving images like this: the original Doctor Who polished up and unleashed with modern production values. It almost makes the convincing argument to give this era the same "remastered" CGI treatment given to other Sci-Fi shows and experimented with on the Dalek Invasion of Earth DVD. It's so convincing, in fact, that this same cover was awarded the distinction of being the best magazine cover ever in the history of Great Britain...