Spearhead From Space.
Doctor Who moves into a new decade, in colour, and with a new Doctor / Companion partnership (the only time there is no companion to bridge a regeneration - until The Eleventh Hour).
There is a link to the past, however, with Brigadier Lethbridge-Stewart and UNIT.
The Doctor begins a lengthy exile on contemporary Earth.
It is revealed that the Doctor has a binary cardiovascular system (that's two hearts to you and me).
He adopts the name John Smith for the first time (it being given to him by Jamie in The Wheel In Space).
Despite it not being his first story for the programme, it is certainly the first of the really great Robert Holmes stories.
The Autons and Nestene Consciousness are introduced.
It is made entirely on film - meaning that it is about to become the only classic series adventure to get the Blu-Ray treatment.
However - not quite as significant as you might think...
Of course, whilst broadcast in 1970, Spearhead From Space, and the rest of Season 7, are actually the final fling of the 1960's - being written and produced in 1969 - and this particular story still has Derrick Sherwin as Producer.
And whilst the stories were now being made and broadcast in colour, the vast majority of viewers continued to watch on Black & White sets (my own family didn't go colour until 1972).
Robert Holmes had written two earlier stories - The Krotons, which has some good ideas but is let down by some clichéd characterisation and poor production values; and The Space Pirates, which is pure cliché from start to finish. This story is where he finally seems to "get" the programme and he will rarely deliver another dud again.
The Autons will help launch three series - and two new Doctors - and one of the Doctor's companions will be turned into one for a period. (The Daleks will launch more series, but only one new Doctor. However, one of the Doctor's companions is turned into one of them as well).