The erudite Alex Niven wrote a wonderful piece about the first two Back to the Future films, which can be seen (and really should be read before reading this post) here:
I wish only to post an addendum that incorporates the third, and much maligned, instalment of this filmic franchise. I want to pick up on Al’s excellent point about the ‘downsizing’ of history in the first two films – Marty travelling back to only a decade or so before his birth in the first film, and to within his own lifetime in the second – by highlighting how ‘the past proper’ is treated in the final film.
For if the past is a different country, a view espoused by Gary Tomlinson (among others), then at least in 1955 McFly speaks the language, albeit with a different accent. However, the ‘Wild West’ Hill Valley of 1885, is almost presented as ‘cut off’ from the present; it is so long ago that its ability to affect permutations on the present are negligible. Remember that this final film is set only one hundred years from ‘the present’, but it is presented as an isolated zone; the story centring entirely within that world, the causality of actions and their effects in the future world are almost entirely neglected.
A couple of interesting images within the film that help demonstrate this: The first sight to meet Marty when travelling back to this ‘distant past’ is a barren landscape into which jump a group of ‘Indians’ on horseback; a primitive people using primitive transport, totally alien to McFly. This is a past inhabited by ‘difference’ and strangeness. Although these Indians are not seen again within the film, they serve their purpose as the original signifier of ‘otherness’ in American culture with startling apparentness.
The second running theme is the notion of being ‘stuck’ in this past. Whereas Marty can travel with some ease between epochs (most notably in the second film), flitting between ‘past’, ‘present’ and ‘future’ (as defined by the limitations of popular culture; 1955, 1985 and 2015 respectively), in the third film there is a repeated notion of being stuck, of not being able to connect ‘distant past’ with the other three epochs. The Doc is initially trapped in the past, Marty comes to rescue him, but cannot get back (no gasoline!). There is not the same notion of interconnectivity between past and present. The story is self-contained within the time period. Not only this, but at least within the main body of the story (ignore the concessionary end where the Doc has to be made to return as a way to bookend the franchise), the Doc has to choose to stay in the past. Even though he and Marty perpetually disrupt ‘the future’ by going to ‘the past’, there is very little stopping them from going there. However, once the Doc goes to the ‘distant past’, he is forced to stay there, there is no way back.
Finally is the image of the landmark town clock, so central in the first two films, is being built in the third. ‘Time’ is literally just being created in this final film; it is the beginning before which, we are led to believe, it would be impossible for Marty to travel.
Perhaps played out in this third film – which I admit is a poor cousin of the first two – is not only the “end of history” that Al talks about, but the beginning of history; history downsized – or maybe compartmentalised into ‘our history’; the complete history of American popular culture. So, having given us the beginning of pop culture, the peak of its power and is potential outcomes (both dystopian and utopian) in its treatment of 1955, 1985 and 2015, the final film shows us the original beginning, the origin; before this there was nothing.
The tagline to the film “They saved the best trip for last… but this time they may have gone too far” sums up that feeling of early 1990s reflexivity in popular culture. For pop culture, 1955 becomes the first point in a tangible ‘past’ from which to glean influence. One hundred years is “too far”. The only way McFly can hope to make sense of it is as ‘Clint Eastwood’, an actor from within his purview. He engages in an act of contemporising this ‘distant past’ (and perhaps vicariously for the viewer) by converting 1885 in to a 1960s Clint Eastwood film; much more manageable – for Marty and the audience. This conversion of distant into tangible past probably explains, at least in part, some of the (deliberate) anachronisms within the film; the arcade-style game Marty plays, ZZ Top as the featured band, the ‘invention’ of the bullet proof vest.
In all this, maybe the crucial question is: ‘why are third instalments of film trilogies always the shittest?’
 I use the word Indians here, in inverted commas, as the image is so stereotypical of the cowboys and Indians genre, to even try and call the people presented in Back to the Future part 3 ‘Native Americans’ would be inaccurate.