I don't want to split things quite that far to be honest. What is a "situation" if it's not a plot?
Well, generally - in a novel or a movie, say - the plot would be
what happens when the protagonist engages with the situation.
Plot is quite simply the events that occur in a story.
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I really don't see the need to cloud the issue when plot is a perfectly serviceable word here.
For me, the need to "cloud the issue" results from a desire to be clear about which participant in the game has authority over what sorts of events.
Because there a distinct group of participants - the non-GM players - who have a special sort of control over the protagonists, I think it is helpful to distinguish
those events in the story which create a context for protagonism and
those events in the story which are the expression of protagonism.
Drawing this distinction has relevance for practical techniques of play, too, as well as for avoiding railroading. In a sandbox, for example, the events that provide the context for protagonism might be determined by rolling on random tables, or by decisions made in the course of world building ("status quo" encounters). Whereas in my personally preferred approach to play they are determined by deliberate choices on the part of the GM having regard to the particular players at hand, and the particular PCs that they are playing.
(This also shows the significance of the difference between "setting" and "plot". A sandbox game relies upon a setting, whether predetermined or encoded in random tables. A BW-style situation-driven game doesn't need a setting in the same way. I get frustrated when world-building sandboxers misunderstand, or ignore, how a situation-driven game is played. But I think it is equally mistaken to ignore the distinct role that "setting" plays in a sandbox.)
What I (and I think Hussar) call plot are "events in a work of fiction." As Hussar pointed out, the rockslide is a 'work of fiction.' My point is that whether the fiction was determined directy by the DM or his choice to use a random table does not make it any more or less a work of fiction. Thus, the rockslide, as a work of fiction, is plot.
The definition you cited did refer to "the organisation of events in a work of fiction". Very crudely, this is "what happens next". Deciding who, in an RPG, gets to decide what happens next, is not a small thing. It's one of the most fundamental issues of RPG play.
The organization is what I refer to when I say the DM can choose to have the event occur because he believes that would be an occurance to happen during mountain travel or because he chose to use a random chart. To me each DM is organizing via his own preferred method, events of plot in his fictional world.
There are no doubts that a GM can adopt different approaches to framing situations. As I've indicated above, these different approaches can suit different broader playstyles.
What is key, in my view, is
who gets to decide what happens next. Once the GM has decided that a rockslide is occurring, who gets to decide how the PCs respond? And to what extent must that response be taken seriously in framing future situations?
Classic D&D doesn't use this sort of language, but it clearly cares about the issues. If the players decide that their PCs consume certain resources in dealing with the situation, for instance, then those resources are not available for future use, and the GM must frame future scenes having regard for that - for example, if the PCs decide to shelter under an overhang while leaving their mule to its fate, then the GM is precluded from beginning the next bit of play by saying "As your mule continues up the mountainside, . . ."
In the Burning Wheel, if the players succeed at a check in the course of dealing with a particular event, then the results of that check stand. The GM can't just override it, either by fiat, or by calling for recheck after recheck until a check is failed. (Likewise for a failed check - a player can't check-monger for success, but must find a new way forward for her/his PC.)
Because, as a general rule, the GM cannot know what resources will be deployed and consumed, what checks attempted, and of any checks attempted whether they will succeed or fail, the GM cannot, as a general rule, know what the upshot of any given event in the fiction will be. And therefore cannot know what will come next. This is at the heart of the way in which both classic D&D, and a modern game like BW, avoid railroading (although in other respects they are oviously very different games supporting very different playstyles).
Conversely, a GM who has
already decided what will come next, and who ignores the upshot of the action resolution mechanics, or who introduces new elements into the fiction in order to render those upshots irrelevant (eg the PCs lose their mule on the mountainside, but the GM's predetermined plot calls fro the PCs to have a mule, and so a fresh mule is discovered wandering aimlessly on the very next ledge), is in my view tending to render the players' choices irrelevant. Those choices are contributing colour ("Yep, Muley the trusty mule was swept away by a rockfall - but here's Muley the Second!") but not much else.
I am happy to accept that this can be a matter of degree - for example, if the GM is adopting a Roads-to-Rome approach, but the manner in which the players' choices bring their PCs to Rome has a significant impact on how the climax in Rome plays out - not just epiphenomenal colour, for example, but colour that goes to the thematic heart of play; or, perhaps, prior decisions about resource consumption that affect the mechanical parameters for the climactic encounter - then player protagonism might be maintained. But
something still has to be left up to the players, in my view, to get satisfactory RPGing. If it's all predetermined - sequence of events, thematic signicance, resources available for meeting the situation, outcome (except perhaps for the final, climactic challenge) - then what is the role of the players? To "experience the story" and make some local tactical decisions (which ultimately won't matter in any event)?
In the end, I don't think I disagree with you (VB) or Hussar that their are events in, or elements of, the fiction over which the GM exercises significant control. But I think by calling it all "plot" you are eliding some issues that are at the heart of satisfactory RPGing, and which different approaches to play - sandboxing, situation-driven play, adventure-path play - handle in very different ways.