'But Verity Lambert, Anthony Coburn, Waris Hussein and Co. took this off the wall idea and made us believe in it. We believed in this utterly bizarre idea because it was grounded in everyday reality, the reactions of our everyman characters of Ian and Barbara were what our reactions would have been if we had stumbled across this impossibility.
There is nothing about this first episode which is trying to be clever or funny; it is just good honest drama and immediately sells Doctor Who as a believable sci fi concept. But it could have been done differently, the producers could have decided to make everything wacky which is pretty much what Moffat does today and then the credibility is gone.
So for the first few seasons Doctor Who goes backwards and forwards in history and out into space, any viewer with a bit of imagination can believe these things are going on, and outside our normal everyday world is a whole universe of adventure. Later things get a bit more complicated because Doctor Who starts to visit contemporary Earth a bit more and aliens start to invade. The problem is that any viewer knows that there hasn't been a worldwide alien invasion so Doctor Whocould lose that connection to reality. The producers therefore do the sensible thing and most of the alien invasions occur in the future or in isolated areas. With a stretch of imagination we can still believe in the reality of Doctor Who and that everything occurs just out of view. Okay a lot of the UNIT stuff is now contradicted, but at least at the time they tried to make their ideas palatable.
The new series has ceased to be believable since probably the first episode Rose back in 2005. In the old days we were slowly edged into the mythology of the series; in RTD's version it comes in one fell swoop: invasions, TARDIS time travel, there is no time for a viewer to be slowly drawn in like there was in An Unearthly Child so it's all chucked in in the space of 45 minutes.'