Back To The Future is one of two films, along with Bill & Ted's Excellent Adventure, which forms the bedrock of what most people of my generation understand about time travel. Back To The Future is the gold standard of time travel stories, ranking up there with the very best of popular science fiction. It's probably the single most popular and well-known time travel story that exists.

And it uses what is probably the most widely-used model of time travel. This model is so widespread and effective that it forms a kind of universal canon. Most people, even people with no real interest in time travel, seem to have picked the rules up through osmosis. It almost seems to be the default model of time travel in most time travel fiction. It can be safely used as the basis for time travel stories, with a relatively minimal amount of explanation.

Back To The Future didn't invent the model, of course, not by decades, but it's the one that I keep coming back to and it's the one that everybody remembers.

How did Back To The Future get to this hallowed position in the science fiction canon? Because it was insanely popular, obviously, but how did it get to be popular? Well... I think hindsight is dangerous in its clarity sometimes. This is a great film, but a great film is not automatically a popular film. It's extremely easy to look back on popular and unpopular films and circle the "obvious" shortcomings of one film or positive qualities of another which led to a particular box office take; examining the characters, story and direction which gained critical approval... without giving proper credit to, say, the marketing campaign, popular taste at that specific year and month, and wild dumb luck. So I don't like to speculate.

Similarly, a popular film is not automatically a memorable one. So: what I think sets Back To The Future apart from other stories using the same model is its fine art of progressive disclosure.

Time travel is a difficult thing for a typical filmgoer to wrap their head around all in one go, and Back To The Future's model is far from the simplest there is. But the model is unfolded gradually, revealed to the audience (through its surrogate, Marty) as a discrete series of new developments. Each bullet point arises intuitively and inevitably from the previous ones, such that by the end of the film, without even realising it, the viewers have developed for themselves a full and functional understanding of what is or is not possible. The film is practically a textbook about its own universe.

Furthermore, each intuitively obvious bullet point is also clearly necessary to the story, because the alternative would be a different story with completely different themes, or even no story at all. The two are coupled: the model exists solely in order to support the story, and the whole story arises naturally from the model. Exactly which concept should come first in the writing process isn't important to the end product, although I believe that in this case it was the story. But the point is that story and model should be tailor-made for one another, they should fit like a hand and a glove.

This is not only true of time travel stories. This is a rule which can be applied to all science fiction, and arguably to all fiction not set in the present day of the real world - and even much of the rest. Every story is set somewhere, and that setting is the deliberate choice of the creator, even if it was picked by default. If the story and the universe conflict with one another, this becomes jarring, and breaks immersion, and causes the reader/listener/viewer to start thinking about the wrong things.

The model

Models of time travel can be organised into a kind of complexity hierarchy, starting with "there is no time travel" and moving up to "fixed history", where time travel is possible but nothing can ever be changed, à la Bill & Ted. Back To The Future is at the next step up on the hierarchy, and presents a "modifiable history". This is the first model where we have to start dealing with the possibility of going back in time and changing history, which means it's the first model where changing history can become a plot point in the story itself, as opposed to using time travel as a framework for some other kind of story. It's also at around this time that we start dealing with multiple timelines, although Back To The Future only lightly touches on this concept, saving that for the sequels.

The model unfolds like this.

1. Yes, you can go back in time.

   This is obviously necessary because without it, there is no time machine and the story is not really about time travel.

   This is established nice and early in "Temporal Experiment Number One", with Einstein the dog in the Delorean's front seat. From a cinema-goer's perspective there is no substantive difference between travelling forwards and backwards in time, although anybody with a [chuckles indulgently, adjusts glasses] rudimentary grasp of relativity will understand that only travelling backwards in time actually violates causality or necessitates changes to the laws of fictional physics.

2. But it's difficult.

   It has to be - otherwise, there would be time travellers all over the place, right? Otherwise "go back in time and fix everything" would be the kill-all, plot-breaking, story-destroyingly straightforward solution to every problem that could possibly come up, whether that problem arose directly, indirectly, or not at all from time-travelling.

   For this reason, time travel is deliberately made difficult in nearly every time travel story, and in my view these limitations often characterise and differentiate those stories. In Back To The Future the difficulty involves specific requirements of energy (plutonium/lightning) and speed. These are good choices for making the action more exciting to watch. As is well documented, early versions of the screenplay had the time machine as a stationary refrigerator, which would have had nowhere near as much style as the Delorean.

3. Having travelled back in time, you can change a few things, and that's fine.

   Chaos theory would suggest that as soon as a single molecule shifts out of place in 1955, all of future history would be thrown off-track. Small changes would snowball rapidly, all around the world. Marty would never be born (or maybe someone else called Marty would be born instead at about the same time), and therefore would immediately be erased from history. Depending on how rapidly the universe responded to such changes, Marty could disappear completely before he even hits the scarecrow in old man Peabody's field. Indeed, the universe could very plausibly instantaneously implode under the weight of paradox.

   This does not happen. Which is a good thing for storytelling purposes. If the universe immediately fractured into pieces there would obviously be no film - or worse, a film which would leave most of its viewers deeply confused and dissatisfied, no matter how carefully their time travel geek friends explained to them, afterwards, what had just happened.

   In fact, even better, nothing happens immediately. Even given the grace period of the "ripple effect" which we see later, in theory there would be nothing to stop Marty's limbs from starting to fade out right away. But this would make him and the audience confused and frightened, at a point in the story where the simpler concern of "being suddenly in 1955" is still front-and-centre. 1955 is a powerfully unfamiliar environment for a kid from 1985, let alone for any of us watching from the distant future present day of - at the time of writing - 2015. We need to slow things down a bit so we can get used to 1955, which is going to be the setting for most of the film.

   And so we deliberately construct our model so that it supports this. Marty is able to stumble around Hill Valley square for a while, comparing and contrasting it with 1985, meeting some significant figures and even boneheadedly kicking off a sort-of causal loop ("That's right, he's going to be the Mayor!"), all without getting into any specific trouble. The universe doesn't seem to mind too much at first that Marty's been displaced - it seems that most of history is reasonably resilient to small perturbations.

4. But some moments in time are much more significant and delicate than others.

   When normal, non-insane people think about history, they often think about critical moments in history; moments which, if they turned out differently, would have pitched all of history onto a completely different track. This is kind of a first-order time travel story premise - your "kill Hitler, save Kennedy, prevent 9/11" moment. It's obvious and logical and easily comprehensible: "What if X didn't happen, and not-X happened instead?" is a far easier prospect to entertain with "big" events whose immediate consequences were very obvious.

   It's entirely possible to build a second-order story where we backtrack up the chain of causality a little, reaching a relatively "small" feeder event - one which, if altered very slightly, would have the chain reaction of preventing the "larger" event. This is quite easy to engineer, Rube Goldberg-style. A missed bus, due to a sandwich which took too long to eat, because of undesired mustard, because of a misheard order, because of a dropped plate, because.... In many stories, one specific feeder event is pinpointed as the sole cause of everything, to the exclusion of all the millions of other feeder events. This might be a sensible storytelling approach if you're trying to make a point about the significance of the mundane, or that any seemingly minor thing you do on any day can in theory have eternal consequences.

   A third-order story recognises the greater, more valuable, more actual truth: that every seemingly minor thing you do on every day absolutely definitely has such consequences. As a logical consequence of this, every single moment in history is of precisely equal significance.

   All of this, though, is non-intuitive, and confuses the issue. It's perfectly possible to tell stories in these frameworks, but these stories can turn out confounding, abstruse and weird, putting more of a burden on the viewers to follow and think and understand than to just relax and assume what seems logical. For a normal, non-insane person, it's much easier if most of history is "safe", but some events are "significant".

   Then we can take the next logical step:

5. If you upset one of those critical moments, bad things can happen.

   And then at last we get to the real conflict. There must be some kind of conflict. In order for this to be a real time travel story, it has to be something which would not be possible without time travel's help. And in order for this to be a real modifiable history story, it has to be something involving modifying history - otherwise we would pick a different model. So, Marty disrupts the fateful first meeting between his future father and mother, and she falls for Marty instead... and his siblings start disappearing from his photograph, and soon he will too!

   Back To The Future is quite smart - or rather, dodges a classic pitfall - because it lowers the stakes while simultaneously raising them, by making the stakes highly personal to the protagonist. Marty isn't the most important person in the world; maybe all the universe isn't likely to self-destruct because some Californian kids were never born. But Marty himself might cease to exist, and by this point in the film we care enough about him that we don't want to see that happen.

   So you've changed history, in a way which is quickly, visibly bad...

6. But you can set things right! (Approximately right will do.)

   As opposed to: "All is lost; there is no way to ever set things right".

   Even if we allow Marty to exist in his own past - maybe in a forked timeline of some kind, with no constraints on its future and no paradox kerplosion - the problem of chaos theory is that all events in history are, once you go back a very short while, totally dependent on chain reactions from all other events in history. Changing a single one of them results in new weather, and new humans being born, and history flipping on its head. This is easy to justify mathematically and scientifically, and makes it annoyingly difficult to tell compelling time travel stories, because even the slightest change to history can never, ever, ever be "set right".

   In Back To The Future we ignore or discard this problem. There's a clear, explicit "good enough" win condition, and some wiggle room in reaching that objective. It's not hopeless. Because, again... if it were hopeless, there would be no film.

7. Although, setting things right is difficult too.

   It would be very possible to go too far in the opposite direction from "it's utterly hopeless". Here, you would end up with a universe which almost has a mind and a will of its own, where Fate is real, and the world has a "specific way it should be", with cosmic forces constantly trying to push Lorraine and George back together. Were that the case, all Marty would need to do is leave, or perhaps be forced to leave by events beyond his control, and everything would be fine.

   Which would be dissatisfying in the extreme. It's very, very important that Marty solves his own problem. Or, as we see in the climax of the story, he tries - but eventually, it's George's problem, and it's George who solves it.

8. And then you can go home.

   Because a happy ending is necessary.

So what we end up with is a spacetime continuum made from discrete major events, joined together by relatively minor ones. This universe permits a certain amount of non-disruptive "time tourism", but reacts badly when certain sensitive points in history are altered. Luckily, this universe isn't so fussy about the details that it instantly swallows you whole if a single electron is displaced. More luckily, this reaction is slow enough that there's room to set things right, if you can.

And the story structure which falls out of this model is the story structure where that's exactly what happens. Our protagonist interferes with the normal course of history, with dire consequences... which he is eventually able to set right.

Words I'd use to describe this formula: powerful; versatile; intuitive; widely-understood.

So, all this essay really covers so far is the model that Back To The Future uses, and the specific conflict and story which rises up naturally out of this particular model: the A-plot. What else is there?

Well, loads. The film is like a Swiss watch, even staying only on the time travel-based plot threads.

There's the causal pseudo-loop whereby Marty invents skateboarding, which we can acknowledge and carefully set aside as fairly unremarkable. There's another loop involving Mayor Goldie Wilson, as already mentioned.

There's another loop whereby Marty invents rock & roll music, and this is much more significant on a personal level because it signals a turning point in Marty's confidence about his own musical abilities. At the start of the film he's frightened that his demo tape will be rejected. He later meets his young father, who has similar creative neuroses, which set Marty's own fears into perspective. But then he gets up on stage and plays "Johnny B. Goode" - and the crowd goes wild for it. He does this without even thinking about it. It's "an oldie" - he already knows the song is going to be a hit. From one perspective it is simply another cheap little causal loop, but from another it's the moment when he realises that he can really can be - no, he is a rock star.

There is, of course, the enormously significant B-plot: the storyline referred to in the title, whereby Marty is stranded in an unfamiliar past era with almost no hope of getting Back To The Future. This storyline is absolute dynamite all by itself, in its conception and execution, and in how it ties into the A-plot by giving Marty serious time pressure to get his parents together before the thunderstorm when he needs to leave. But that kind of story structure, of being stranded in time with no way to get "home", is not really unique to this modifiable-history model (and nor is the "Marty invents rock & roll" thread, come to that). You can tell a story like that in a fixed-history model quite easily.

Hell, if you want to spend time examining the travails of an interesting, stranded protagonist, you can stick them on a desert island in the present day. You don't throw time travel into your storyline for no reason at all. You do it because there's a particular thing in the past or the future that you want to explore, and because you need the model to support that exploration. Here, it's the generational differences between the Fifties and the Eighties - or, as we discover, generational similarities. That's why I consider the Lorraine/George/Marty storyline to be the A-plot of this film.

The final time travel-related plot thread is, I guess, the C-plot: Doc is gunned down by terrorists in the first act before Marty leaves 1985, and Marty wants to somehow rewrite that small piece of history in order to save him. This is a difficult problem, hovering in the background during most of the 1955 scenes. Marty is caught at the junction of two conflicting desires, one to set established history straight to put his future parents back together, and one to alter established history to save his friend.

Even not knowing what truly happens to him in the future, Doc refuses to hear any kind of specific warning from Marty, saying that no one should know too much about their own future, and that the consequences could be disastrous. Doc's resistance is quite rational, because both of them have seen the possibility of erasure from history with their own eyes. The conflict arises because Marty knows full well that this is no worse than what actually happens, and the Doc has absolutely nothing to lose - and none of history beyond that point is known, so nothing can be lost from altering that one moment.

(There are broader implications which the film ignores and which I think it's reasonable for us to ignore too. Specifically, there are numerous other significant historical events coming up between 1955 and 1985, which Doc can't possibly foresee. It would be rational if Doc was in fear of unwittingly disrupting one of these, thereby endangering the existence of many people other than himself. But I feel happy discarding this consideration because, as I mentioned above, the whole film is highly personal - Doc seems to have no family or friends beside Marty, so it's reasonable to pretend that Doc is the only person at risk from his own foreknowledge.)

Of course Doc eventually decides he can trust his friend, and everything turns out fine.

This storyline somewhat reinforces the stern warning from the A-plot, which is that "there's a way that history ought to be", while being the first thing in the film to hint that the way history ought to be isn't necessarily the way that it is. Later, Marty gets home, and finds that the new status quo is much better than it was when he left. And in the final scene, Doc returns from 2015 with nothing else on his mind but setting right an unspecified future gone bad.

Except that the time machine is a dangerously powerful tool for the "righting of wrongs", as we'll soon see...

Overall Back To The Future is a film whose core concept is that while the world alters radically from generation to generation, your parents are really not so different from you. The film selects and tailors a model of time travel to explore this concept, and then goes every conceivable extra mile - laying the whole concept out brilliantly for the viewers; harnessing the core model to drive all kinds of other stories, both dramatic and comedic; never wasting a single word of dialogue; driving iconic action sequences and somehow also finding time for a genuine emotional story with a lot of heart.

I think the film may be diminishing in its effectiveness now that the eras of 1955 and 1985 have both become so alien compared to the present day. 1955 was always supposed to be a disconcerting change of pace. And the opening sequence of the film with Marty on his skateboard is almost prescient - it introduces us to the era of 1985 as if we were strangers to it, in almost exactly the same way that the film later introduces us to 1955... but even so.

It doesn't matter, though, because the Delorean is forever. Have I mentioned the Delorean? Unquestionably the greatest, most iconic, coolest-looking time machine ever imagined? Oh my goodness, the Delorean.