One useful way of thinking about plot comes out through character. Consider how in any given scene (or interaction) between two human beings, the human beings will generally have things they want, and those things are probably not identical. So some of the tension and interest of the scene comes from the quiet conflict between their desires.
Consider Hamlet for an example -- when Hamlet talks with his uncle early in the play, Hamlet is trying to figure out what has really happened -- was his father murdered, and did his uncle have anything to do with it? While his uncle is trying to figure out what Hamlet has figured out about what actually happened. They're working at cross-purposes, and that lends energy to their oblique dialogue. It's a type of plot tension.
Similarly, in Hemingway's "Hills Like White Elephants", the two characters are discussing the same problem, but they want different solutions to the problem -- and more specifically, they each want the other person to say that their solution is the right one. From that quiet tension of their mismatched desires comes the entire plot and heartbreak of the story.
If you haven't read either of these, go read them.
So when you're thinking about plot, think about what your various characters are trying to accomplish in their lives at that point. These don't have to be big things, and the characters don't even have to be particularly active about attempting to accomplish those things (although if they're *really* passive, you'll run into other difficulties). Do they want to live a more interesting life? Do they want to get the girl? Do they want a good night's sleep? What's getting in the way of their accomplishing their goals, satisfying their desires? How do they try to get around those obstacles?
One of my teachers, Nicola Griffith, said that the key to getting readers to empathize with your characters was to have your character want something badly, anything. It doesn't matter what it is, because all of us know what it is to want something badly, and we will empathize with that struggle. If you then have multiple characters in your story and scenes, each wanting things that are not identical, then you have innate conflict and an organic path to plot tension.