Landmarks No.1

An Unearthly Child.
The first in a regular series - of 50 landmark stories - as promised / threatened last week.
A bit of a no-brainer for the start of these milestones, but it is significant for being more than simply the first ever adventure.
The Doctor and the TARDIS are introduced - elements which still form the core of the programme five decades later. We know a little more about him, and a little more about her - but not that much considering the time-span.
With a lot of pilots / series openers, the principle character is introduced and we generally get a lot of information about them and the people they live / work with.
With the Doctor, we learn that he is an alien, rather than someone from Earth's future or an eccentric inventor. He has a grand-daughter named Susan, has been on Earth for a few months, and can't return home. And that's your lot. We don't even have a name.
As events unfold in prehistoric times, we don't even know if he's a good guy or not - seeing as how he seems on the point of killing Za in order to make an escape.
Other items of significance include the fact that Ian and Barbara are clearly the first ever companions to travel in the TARDIS, other than Susan.
It is in this story that the TARDIS chameleon circuit first breaks down and the ship becomes fixed as a Police Call Box.
TARDIS design elements such as the roundels and the hexagonal control console are established.
We are informed that TARDIS is an acronym - and are told what it stands for.
It's the only story in which the Doctor is seen to smoke.
An Unearthly Child also marks the beginning of the first ever story arc in the programme - something which is all but compulsory today but was rare in the early days. The arc in question is the attempt by the Doctor to get Ian and Barbara back home again, and it runs through to The Chase 18 months later.
Episodes 2 - 4 represent the first of the purely historical stories, which will be a mainstay of the programme for the next three years, alternating with the futuristic / sci-fi stories (more of which next time...)..