The main plot revolves around two story lines that come together at the end of the story with some smaller story lines interwoven. The two main threads are the children's fear of and fascination with Boo Radley, and the trial of Tom Robinson. Those come together as one at the end when Boo kills Bob Ewell to protect Scout and Jem. Mrs. DuBose, her crankiness and her determination to free herself of her morphine addiction, is one of the less important lines. Aunt Alexandra and her prejudice is another line. The summers spent with Dill are a feature of the story intermixed with the Boo Radley and Tom Robinson lines. I'm not sure how much detail in plot events you want, so I suggest that you check out the links listed below, especially the link to the one-page summary.