I've seen some discussions about plot here and there recently, at Romancing the Blog among other places. I've talked about plot before, but usually in support of something else, or on a comment on another blog. So here's my take on what your plot is:
Figure out who your protagonist is. Figure out what she wants. Figure out why she can't have it. Figure out what she does to try to get it anyway. There you go.
The "what she wants" part is your goal.
The "why she can't have it" part is your conflict.
The "trying to get it anyway" part is the meat of your plot, the action, the story, the "what happens" between the beginning and the end.
The climax is when the protag either gets what she wants, or realizes/accepts that she can't have it after all and deals with that.
Protagonist ==> Goal ==> Conflict ==> Trying To Achieve Goal Anyway ==> Climax
This is the bare skeleton of a plot; you can hang whatever flesh you want onto it, add skin, moles, freckles, a tattoo, a sleeve, a belt, etc. But a story has to have a plot or it's not a story, so you need at least the basics, at least one main plotline. (And in a short story, one is usually all you have room for.)
Knowing what your main plotline is gives you a solid baseline while you write. Particularly with longer or more complex stories, where you have two or more intertwining plotlines, and/or a number of subordinate plotlines, they all need to be heading in the same direction ultimately, even if they start out far removed from each other in time and space, each with its own set of characters. They need to come together in the end and support some one storyline, to help (or hinder) the resolution of the main plotline; if they don't, then they don't belong in the same story. (Check out the Niven/Pournelle collaborations for some great examples of how to start out with half a dozen or more far-flung sets of characters and goals, and wind up with everything woven neatly together in the end. Lucifer's Hammer does this brilliantly.)
Even if you're a pantser (which I am), knowing what your main plot is gives your story a strong spine, and gives you as the writer a failsafe ruler to use when you're judging whether this or that subplot or supporting character or gimmick, or conversation or description or chunk of narrative, belongs in the story. Does it ultimately support the main storyline, even if only peripherally? If not, then chuck it.
Another way to think of the main plotline is as a six-lane interstate going from the beginning to the end of your story. You can add other roads -- state highways, frontage roads, meandering surface streets, dirt tracks through abandoned fields, whatever -- but they all need to connect back to the interstate eventually. Anything wandering off completely on its own, unconnected to anything else, doesn't belong on the map of your story.
Note that having a plot doesn't mean your characters can't ever just sit down and talk.
Having a plot doesn't mean your characters can't ever just sit down and contemplate their problems within their own minds.
Having a lot of action does not mean you necessarily have a plot. Random fire-fights or car-chases do not make a plot, while actionless conversation just might. One of the most common misconceptions about plot I see in discussions around the web is the idea that Plot <=> Action, and that if you don't have a physical fight or a mystery to solve or something like that in your story, you don't have a plot. That's not accurate at all.
Plot is about your characters trying to solve one or more problems, trying to get what they want despite obstacles. If that frantic speedboat chase doesn't help your protag get what she wants (part of the solution), or directly prevent her from getting what she wants (part of the obstacle), then it doesn't further your plot. If leaning over the fence and having a conversation with that annoying neighbor helps your character solve his problem (part of the solution), or at least shows him more clearly what the problem is (helping define the obstacle), then that conversation does further the plot.
Goal ==> Obstacle ==> Solution
That's plot. How you flesh it out is up to you.