Currently living in New York City, Jean takes the train down south to Maycomb to visit her family for a few weeks. A lot of things are different now that she's grown up: her aunt Alexandra now lives with Jean's dad Atticus; Atticus has arthritis now and has trouble using his hands; and sadly, her brother Jem has died some time ago. Jean also has a boyfriend now who lives in Maycomb, but she doesn't seem serious enough about him to marry him. The conflict begins when Jean finds a racist pamphlet that belongs to Atticus, and she gets furious at him for being part of a backwards-thinking, racist movement. Her boyfriend is in on it, too--the two men are part of a group that meets in the courthouse to talk about how to keep black people from earning equal treatment in society. Jean is disgusted by all of this and talks to a few others about this problem (including Calpurnia and her wise uncle), eventually deciding to forgive Atticus and accept that he's just a human capable of making mistakes.
Of course, every reader's summary will be different, because we'll have different opinions on which events are the most important.
And you can always squeeze in more information by creating longer sentences. Join two sentences that you already have by replacing the period between them with a semicolon, for example; this one right here could have been two sentences, but I made it one by using the semicolon instead of the period. That makes room for another sentence if you need it.