neighbor, a recluse calledBoo Radley, whom they have never seen but who seems to be leaving them gifts in a tree outside his house.

When a black man, Tom Robinson, is accused of raping a white woman, Atticus agrees to defend him, even though this means disapproval and even violence against him and his family from the white citizens of Maycomb, some of whom come close to lynching Tom. During the trial, which is witnessed by Scout, Atticus proves that Tom's accuser, Mayella Ewell is lying but Tom is convicted anyway and later dies while attempting to escape from prison.

Mayella's father, Bob Ewell, feels humiliated by his daughter's treatment during the trial and embarks on a campaign of violence and intimidation against the Finch family, culminating in an attack on Scout and Jem from which they are saved by Boo Radley who (probably) kills Bob Ewell by accident but is able to escape the consequences and return to his reclusive lifestyle.

A first-person narrative recounting two years in the life of Scout Finch, To Kill a Mockingbird is a novel of personal growth, social justice (and injustice), and friendship. While the writing employs generous amounts of humor, the core of the book is essentially concerned with the problems of prejudice and cultural bias in the 1930s American South. 

To Kill a Mockingbird is a retrospective view of the process of her own maturation by Scout Finch as she passes from the superstitious child who fears "haints" to the girl who learns that people are often from odd homes with strange values.  In so learning of the diverse thinking of others, Scout comes to understand the differences of people while at the same time she is made aware of the evil that men do"(Julius Caesar).  From her many childhood experiences, Scout acquires an appreciation for the wisdom of her parent and her friend, Miss Maudie, as well as a mature evaluation of her society.

The novel is a story of how two children learn more about the various kinds of discrimination that exist in Maycomb. This is something that they too are involved in, as the discriminate Boo Radley, just as others discriminat him and other characters in the novel. By the end of the novel, however, they are forced to see the dangers of discrimination and are able to overcome their prejudice, as they come to view Boo Radley as a human and a friend and not a bogeyman.

I'll put a slightly different spin on this.

This book is the story of how two children, Jem and Scout Finch, grow up and start to understand the world in more adult ways.  As the book starts, they are young children who do not understand why the world is the way it is.  They look at it from a very narrow point of view.  Through the Tom Robinson trial, as well as through their interactions with such people as Boo Radley, the Cunningham family, and Mrs. Dubose, the children learn to view the world in a completely different way.

Scout and Jem become aware of the prejudice throughout their community and realize Boo Radley is a caring, selfless individual. Atticus teaches his children many important lessons such as, the importance of perspective; being courageous when faced with adversity; why harming innocent human beings is wrong, and how to make the morally correct choice during difficult times.

OR

Harper Lee uses the character of Scout, who is 6 when the novel begins, to tell a story of how her father, Atticus, defends Tom Robinson, and how she explores the existence of her reclusive neighbor, Boo Radley, in the small town of Maycomb, Alabama in the 1930's. Throughout the novel, Scout learns valuable lessons from her father, which include: how to view situations from other people's point of view; the existence of prejudice and injustice in the world; why it's wrong to harm innocent people; what "real" courage is, and the importance of being true to oneself.

The novel tells the story of a young brother and sister growing up in Depression-era Alabama and their own evolution as the lives of others evolve around them. The children learn about the importance of tolerance toward others; that people--and appearances--aren't always what they seem; that hypocrisy and racism is a fact of life that is best observed and not practiced; and that justice is not always served: They grow up faster than most, and their loss of innocence robs them of some valuable childhood years, but they also discover the value of their father's good judgement and that their own life experiences provide them with lessons far greater than what they can learn in a classroom.

Brief Outline of To Kill a Mockingbird by Harper Lee:

• Scout, her brother Jem, and her father, Atticus Finch (a lawyer) are introduced.
• They live in Maycomb, Alabama, during the Great Depression.
• Jem and Scout make friends with Dill.
• Jem, Scout, and Dill become interested in the Radley house, and especially Arthur (Boo) Radley.
• Scout begins school but doesn't like it.
• Jem and Scout find gifts left for them in the knothole of an old tree.
• The children act out stories about Boo Radley, but Atticus tells them to stop and to consider another person's point of view.
• The children sneak onto the Radley property and Mr. Radley shoots at them.
• Jem loses his pants but gets them back mended.
• Dill leaves.
• The children find more presents.
• Mr. Radley plugs the knothole with cement.
• There is a fire at a neighboring house and someone slips a blanket over Scout's shoulders as she watches.
• Jem tells Atticus about the presents and the mended pants.
• Atticus agrees to defend Tom Robinson, a black man accused of raping a white woman.
• Other children bully Jem and Scout because their father is defending a black man.
• Calpurnia, the Finch family cook, takes the family to the local black church.
• Alexandra, Atticus's sister, comes to live with the Finches.
• Dill returns.
• A lynch mob gathers to lynch Tom Robinson.
• Atticus faces the mob down. Scout innocently convinces a man to disperse the mob.
• The trial begins. Jem and Scout sit in the "colored balcony."
• Atticus provides evidence that the defendant, Mayella Ewell, and her father are lying. He proves that the defendant propositioned Tom, and that it would have been impossible for Tom to inflict the wounds on her.
• The all-white jury convicts Tom despite the overwhelming evidence that he is innocent.
• Tom tries to escape from prison and is shot to death.
• Jem loses faith in justice.
• Bob Ewell, the defendant's father, feels he has been made a fool of during the trial and vows revenge.
• Ewell attacks Jem and Scout in the woods. Boo Radley intervenes. He stabs Ewell to death.
• Boo carries Jem home to Atticus.
• The sheriff reports that Ewell tripped and fell on his own knife to protect Boo.
• Scout realizes that Boo Radley is not a monster, but a human being.
• Scout learns to be sympathetic and understanding towards others.