At the beginning of the book she tells us how she spends the summer time with Jem and her friend Dill trying to convince, with all sorts of tricks, their mysterious neighbour, Boo Radley, to come out of his place and let them see his face. Boo Radley lives confined in his parents' house for some behaviour problems he showed in the past, and nobody has seen him for years.
We see, or better said, we imagine this character through Scout's mind and thoughts. At the beginning she is pretty scared of him, but, as time goes by, she changes her mind, showing a sort of pity for this unfortunate man, who, maybe, has decided not be seen by his peers, to avoid their hypocrisy and prejudice.
In the meantime Scout describes her surroundings: the people of Maycomb are easy to classify. By social status and behavior.
She also talks about her father, the lawyer Atticus. The only one in town who seems not to respond to small towns implicit laws: he is an absolutely fair man, who teaches his children that before judging anyone they must spend some time in someone's shoes.
Later, the plot focuses on the main story of the book: the event that will bring to the surface what politeness and good manners have been hiding till that moment--racial discrimination.
A Negro is charged with the rape of a white girl, and even if Atticus manages to objectively demonstrate his innocence, the all-white jury convicts him, just because he's black. That was how it worked those times.
The town is divided into two groups: the one which thinks that Negroes are subordinate and must be kept in their 'place', and the smaller group that wants people of any color to be considered human beings with the same rights.
So, while she wants to tell us how Jem got his arm broken at the age of 13, she ends up painting a detailed picture of Deep South American society in the 30's.
Thanks to the first person narrator we feel exactly what she feels: curiosity, fear, sorrow, powerlessness, anger.
Furthermore, the author gets to completely suck us into this society that looks so far away, but that in the end is still alive nowadays, everywhere.
It's an enlightening book that every child (and adult) should definitely read, to remind ourselves that every day is a good day to try to understand people instead of judging them at first sight or according to what others say of them.
Prejudice will kill the mockingbird.
Mockingbirds don't do one thing but make music for us to enjoy... but sing their hearts out for us. That's why it's a sin to kill a mockingbird.
More about racial prejudices: The Lions of Little Rock