Landmarks No.6

The War Machines.
What, you might be thinking, is so significant about this liked, but not loved, late Hartnell era story?
Well, it is the first of the contemporary Earth under threat stories - the forerunner of the UNIT adventures, and the other myriad Earth invasions (from extraterrestrial or homegrown powers).
The Doctor is either using the psychic paper (unseen) for the first time, or he has built up some influential contacts whilst Susan was attending Coal Hill School classes. Perhaps the gentlemen's clubs frequented by the Third  Doctor were originally patronised by the First.
He has easy access to the high security section of the new GPO Tower, and wangles an invite to the press launch for the WOTAN C-Day event. Sir Charles Summer is expecting him - and is quick to offer Dodo and he accommodation.
When the military and the government get involved, the Doctor's presence is readily accepted - thanks to his link with Sir Charles.
All of this presages the UNIT years, when the stories do not need to have the Doctor explaining who he is and getting locked up etc. The Doctor hits the ground running - accepted immediately so that we can all get on with the story.
The psychic paper was invented just for the purpose of speeding things up and doing away with unnecessary explanations.
The present day London locations make a welcome change from studio planets and historical settings.
This story is also the first to make use of a real-life news reader (the late Kenneth Kendall) to add verisimilitude to the events taking place on London's streets. All Earth invasion stories these days make use of this gimmick (most recently The Power of Three).
The story also marks a companions change-over - Dodo getting the worst departure ever, and Ben and Polly coming in to soon bridge the transition to a new Doctor.