An interesting challenge. If you want a longer summary, I'd look at the summary section of the enote study guide to this novel.
For now, let me see what I can do with one paragraph:
To Kill a Mockingbird is the story of some defining events in the life of Scout Finch (whose real name is Jean-Louise), events that helped her group up into a good person. The novel focuses on her relationships with her family (her brother Jem and father Atticus and her extended family), her friends (especially Dill and Calpurnia, the family housekeeper who is closer to family) and, eventually, Boo Radley. The major event in the novel is a trial in which Atticus defends a black man who was wrongfully accused of rape.