Landmarks No.10

The Web of Fear.
UNIT and the Brigadier start here. There had been a scant couple of stories set in contemporary times before this - starting with The War Machines and continuing through The Faceless Ones and the early episodes of Evil of the Daleks - all coincidentally occurring at the same time - but this story is where the roots of the UNIT years are really to be found.
London has been evacuated due to the appearance of a strange web-like substance, and then Yeti turn up.
The Great Intelligence is back to try again.
The army set up command in the Underground - off of which is a World War Two bunker near Goodge Street. (If, like me, you are interested in the history of London - especially "secret" London - you will know that the city is built on a number of WW2 and Cold War installations. The entire course of the Victoria tube line is determined by secret government structures).
Placed in charge of the military operations is a Colonel from a Scottish regiment by the name of Alistair Gordon Lethbridge-Stewart.  He doesn't show up until the end of episode two (his boots being played by an extra) but in part three, Nicholas Courtney joins the action. The rest is history. The future Brigadier will appear with the second, third, first, fourth, fifth and seventh Doctors, making his final appearance alongside Lis Sladen's Sarah Jane Smith in the CBBC spin-off series, and will be name-checked as recently as this season's The Power of Three, where we find that the organisation he helped to found is now being run by his daughter. Looking to the future, Kate Stewart will be back in the 50th Anniversary tale.
Of course, Courtney had failed to appear in the programme back in The Crusade - the role of the Lionheart going to Julian Glover. He had appeared in The Daleks' Master Plan as the ill-fated Bret Vyon. He was nearly "ill-fated" again in this story - originally being offered the part of Captain Knight, who doesn't make it past episode four.
The set up of The Web Fear leads directly to The Invasion, which leads in turn to Spearhead From Space, and some of Doctor Who's most popular stories. Nicholas Courtney becomes one of the programme's most beloved personalities and ambassadors.