That evening, Dill and Jem decide to sneak up to the Radley house where the porch swing creaks - with a scared and nervous Scout following behind them - to "look in the window of the Radley house and see if we can get a look at Boo Radley." The three crawl and squeeze under a high wire fence at the rear of the Radley property and cautiously approach the house. At the ramshackle back porch, Jem creeps up the noisy steps toward one of the windows and tries to peer in. Suddenly, a large shadow of a man appears, moves across the porch and looms over him - crossing over his body. A menacing hand reaches out. All of them cover their eyes and cower in sheer fright, but then the image retreats just as mysteriously. They leap off the porch and back under the fence, but Jem's overalls get snagged in the wire mesh - he discards his pants (with Scout's and Dill's assistance) to get untangled and free and then runs toward home in his underwear.

To avoid being whipped by his father for not having his pants, Jem disappears through the fence to go back and retrieve his abandoned overalls - it is a tense few moments as Scout slowly counts to fourteen, hoping that her brother (off-screen) will return. Her counting is interrupted by the sound of a shotgun blast. Jem bursts through the hole in the fence, just as Mr. Radley appears with his shotgun on the street, telling Atticus and Aunt Stephanie that he "shot at a prowler out in his collard patch."

With summer ending and Dill returning to his home, school begins. Tomboy Scout must give up her overalls for a dress. She awkwardly pokes and tugs at it on her first day of school: "I still don't see why I have to wear a darn old dress." In the schoolyard, her natural outspokenness, honesty and candor get her into a tussle with Walter Cunningham Jr., (Steve Condit), the seven year-old son of the farmer from Old Sarum. She rubs his nose in the dirt. Scout explains to Jem, as he restrains her with all his might, why she took out her frustration at the teacher on the poor boy:

He made me start off on the wrong foot. I was trying to explain to that darn lady teacher why he didn't have no money for his lunch, and she got sore at me.

Jem promises that his "crazy" sister won't fight with him any more and then invites young Walter over to the Finch household for a dinner of roast beef (corn bread, turnips and rice) rather than his usual fare of "squirrels and rabbits." During the meal, Atticus explains the responsibility his father taught him in using his first gun when he was thirteen or fourteen - and how it is 'a sin to kill a mockingbird' - a songbird that harmlessly exists only to give pleasure:

I remember when my daddy gave me that gun. He told me that I should never point it at anything in the house. And that he'd rather I'd shoot at tin cans in the backyard, but he said that sooner or later he supposed the temptation to go after birds would be too much, and that I could shoot all the blue jays I wanted, if I could hit 'em, but to remember it was a sin to kill a mockingbird...Well, I reckon because mockingbirds don't do anything but make music for us to enjoy. They don't eat people's gardens, don't nest in the corncribs, they don't do one thing but just sing their hearts out for us.

Scout watches Walter as he liberally pours thick syrup all over his meal. Appalled and disgusted, she hurts his feelings: "He's gone and drown-ded his dinner in syrup and then he's pourin' it all over." In the kitchen, the black housekeeper Calpurnia (Estelle Evans) gives Scout a lesson about manners and tolerance:

That boy is your company. And if he wants to eat up that tablecloth, you let him, you hear? And if you can't act fit to eat like folks, you can just set here and eat in the kitchen.

Scout is sent back to the table with a smack on her rear. With a caring understanding of the mysteries of childhood, Atticus finds Scout on the slatted porch swing hanging on a rusty chain and sits next to her. He listens to her share her feelings about the crisis on her first day of school, and her criticisms of her teacher. Without talking down to her, he eloquently, simply, and tenderly presents her with an invaluable lesson on how to accept the differences between one human being and another:

If you just learn a single trick, Scout, you'll get along a lot better with all kinds of folks. You never really understand a person until you consider things from his point of view...Until you climb inside of his skin and walk around in it.

With paternal wisdom, he also tells her about the meaning and value of compromising:

Atticus: Do you know what a compromise is?
Scout: Bendin' the law?
Atticus: Uh, no. It's an agreement reached by mutual consent. Now, here's the way it works. You concede the necessity of goin' to school, we'll keep right on readin' the same every night, just as we always have. Is that a bargain?

As the scene continues, the adult Jean-Louise - in voice-over - praises her father:

There just didn't seem to be anyone or anything Atticus couldn't explain. Though it wasn't a talent that would arouse the admiration of any of our friends, Jem and I had to admit he was very good at that, but that was all he was good at, we thought.

In the next memorable sequence, Atticus proves his Lincoln-esque stature to his children. Although Scout is disbelieving and yells out "He can't shoot" when Sheriff Heck Tate (Frank Overton) hands his rifle to her father, Atticus takes aim with a rifle at a rabid dog moving erratically down the street outside their home. He raises up his glasses a few times on his forehead to see better, and then removes them altogether by dropping them on the street. Jem and Scout are both dumbfounded and stunned when the rifle cracks and the dog flops over dead. The Sheriff tells Jem about the hidden abilities of his modest father who hasn't shot a gun in twenty years: "Didn't you know your daddy's the best shot in this county?"

That night, Jem and Scout join their father as he rides into the country to compassionately talk to his client's family - twenty-nine year-old Helen Robinson (Kim Hamilton), the wife of the man he is defending. While Atticus is in the Robinson house, an unshaven and drunken Bob Ewell staggers toward the car, holds onto it to steady himself, and stares at the two children. Atticus appears and after they face off, Ewell hatefully snarls at him: "You nigger lover." Jem's understanding of the world is altered and he needs reassurance: "No need to be afraid of him, son. He's all bluff." As they drive away, the camera takes Jem's point-of-view as he watches the prejudiced, gesturing and threatening figure standing in the middle of the road. When they return home, Atticus adds: "There's a lot of ugly things in this world, son. I wish I could keep 'em all away from you. That's never possible."

While Atticus drives Calpurnia home, Jem sits on the porch in the rocking chair. He is spooked and terrorized by rustling trees, moving shadows, and the calls of a nightbird. He starts to run toward the Radley place in the direction of his father's car, calling out: "Atticus, Atticus." Realizing it is futile to try to catch up to the car, he stops and turns toward home at the edge of the Radley property, noticing something shiny and reflective in the moonlight - in the hollow knothole of an old oak tree. He sticks his hand in, takes the object out, notices it is a shiny medal, and quickly pockets it before running home.

While Scout argues and fights at school with another boy, this time Cecil Jacobs (Kim Hector), the adult voice of Jean-Louise remembers the incident that provoked her:

Atticus had promised me he would wear me out if he ever heard of me fightin' any more. I was far too old and too big for such childish things, and the sooner I learned to hold in, the better off everybody would be. I soon forgot...Cecil Jacobs made me forget.

Later that afternoon on the Finch front porch, Scout sits with her head buried in her arms. She reveals to Atticus the 'fightin'' words that caused her to beat up another neighborhood boy to defend her father's work:

Scout: Atticus, do you defend niggers?
Atticus: Don't say 'nigger,' Scout.
Scout: I didn't say it...Cecil Jacobs did. That's why I had to fight him.
Atticus (sternly): Scout, I don't want you fightin'!
Scout: I had to, Atticus, he...
Atticus (interrupting): I don't care what the reasons are. I forbid you to fight.

Atticus patiently explains his reasons for making the unpopular decision to defend a Negro - a most-hated and despised person in society, regardless of the consequences:

Atticus: There are some things that you're not old enough to understand just yet. There's been some high talk around town to the effect that I shouldn't do much about defending this man.
Scout: If you shouldn't be defending him, then why are you doing it?
Atticus: For a number of reasons. The main one is that if I didn't, I couldn't hold my head up in town. I couldn't even tell you or Jem not to do somethin' again. (He puts his arm around her.) You're gonna hear some ugly talk about this in school. But I want you to promise me one thing...that you won't get into fights over it, no matter what they say to you.

In the knothole, both Jem and Scout find two carved soap figurines - one the figure of a boy and the other a girl with a crude cloth dress. They realize that the playthings have a resemblance to themselves: "Look, the boy has hair in front of his eyebrows like you do...Yeah - and the girl wears bangs like you - these are us!" Suddenly, Mr. Radley comes from behind the tree and starts filling the knothole with cement from a trowel.

That night, Jem shows Scout the contents of a cigar box after forcing her to promise "never to tell anybody." In it is the growing collection of items that he has found in the knothole - including a crayon, marbles, a whistle, a spelling medal, an old pocket watch, and a pocketknife. And he reveals another secret - when he went back to fetch his tangled britches, they were neatly "folded across the fence - sorta like they was expectin' me."

An adult Jean-Louise comments about the mystery:

It was to be a long time before Jem and I talked about Boo again.

When school finally ends, summer and Dill arrive again. The trial of Tom Robinson is scheduled for the following day, and the defendant is brought back into town from the Abbottsville jail where he was held for safe-keeping. That night, Atticus decides to stand guard outside the town jail because of rumoured agitation from "that bunch out at Old Sarum." Later that night, the three children run from the Finch house toward the jail through deserted and dark streets. From behind bushes in the town's square, they notice a solitary light burning in the distance. While reading a law book under a lamp shade he has brought from home, Atticus is seated in a chair propped up in front of the jail's front door where Tom Robinson is being held. Jem is satisfied: "I just wanted to see where he was and what he was up to. He's all right. Let's go back home."

In one of the most compelling scenes in the film, as the children begin taking a shortcut home, four cars noisily converge on the jail from the Meridian Highway. The children hide and watch from the cover of the bushes. The armed men get out of their cars and surround Atticus - they are a self-appointed lynch mob that has gathered to take justice into its own hands after diverting Sheriff Tate. To get a closer look, the three kids run over to the cars. Scout, in particular, who is oblivious to the danger, pushes her way through the crowd to glimpse her stern-faced father - he immediately fears for their safety. While Jem stands by his father and stubbornly refuses to leave after his father's command, a stalwart Scout faces down the crowd and sees someone she recognizes. She conducts an innocent, uninhibited exchange with Walter Cunningham Sr., and engages him in a disarming, candid, yet humanized conversation. Scout makes him uncomfortable in front of the mob:

I said, 'Hey,' Mr. Cunningham. How's your entailment getting along? (He turns and looks away.) Don't you remember me, Mr. Cunningham? I'm Jean Louise Finch. You brought us some hickory nuts one early morning, remember? We had a talk. I went and got my daddy to come out and thank you. I go to school with your boy. I go to school with Walter. He's a nice boy. Tell him 'hey' for me, won't you? You know something, Mr. Cunningham, entailments are bad. Entailments...(She suddenly becomes self-conscious) Atticus, I was just saying to Mr. Cunningham that entailments were bad but not to worry. Takes a long time sometimes...(To the men who are staring up at her) What's the matter? I sure meant no harm, Mr. Cunningham.

Scout's words cause him to break up the potential lynching. The embarrassed crowd disbands.

The next day, the explosive trial brings scores of country people to town to watch the case unfolding in the rural Southern courthouse. Although the children have been ordered to say home, Jem can't resist being there with them: "I'm not gonna miss the most excitin' thing that ever happened in this town!" The courthouse square is empty, but the courthouse is filled with spectators. Because the downstairs is "packed solid," elderly black Baptist minister Rev. Sykes (Bill Walker) lets the children join the blacks that are consigned to the 'colored' balcony on three sides of the courtroom. They peer over the balcony railing onto the scene below. The jury, seated to the left under long windows, is composed nearly entirely of farm folk who haven't been able to avoid jury duty.

The courtroom sequences are dramatically-filmed. In the opening testimony by Sheriff Tate to the circuit solicitor Mr. Gilmer (William Windom), it is learned that on the night of August 21st, Bob Ewell reported the beating and rape (she was "taken advantage of") of his girl, Mayella Violet Ewell (Collin Wilcox). It is simply the word of two white people against the word of a black man. During cross-examination by Atticus, Tate reveals that nobody called a doctor: "she was beaten around the head. There were bruises already comin' on her arms. She had a black eye startin' an'... - it was her left." But then after some clarification, he corrects himself:

It was her right eye, Mr. Finch. Now I remember. She was beat up on that side of her face...She had bruises on her arms and she showed me her neck. There were definite finger marks on her gullet...I'd say they were all around.

The next witness is Mayella's father Bob Ewell, who testifies that when he came home that night, he heard his daughter screaming and found Robinson on top of her before chasing him from the house: "I seen him with my Mayella...po' Mayella was layin' on the floor squallin'." During cross-examination, Atticus hands paper and pencil to Ewell and has him write his name to discover that he is left-handed. The children watch everything very intently from the balcony ledge. Ewell angrily complains to the judge: "That Atticus Finch is tryin' to take advantage of me. You gotta watch lawyers like Atticus Finch."

Mayella, a "white trash" woman accustomed to strenuous labor, takes the stand next and testifies that she invited Tom inside her yard to do chores. That was when he attacked her in the house:

I was sittin' on the porch, and he come along. Uh, there's this old chifforobe in the yard, and I-I said, 'You come in here, boy, and bust up this chifforobe, and I'll give you a nickel.' So he-he come on in the yard and I go in the house to get him the nickel and I turn around, and 'fore I know it, he's on me, and I fought and hollered, but he had me around the neck, and he hit me again and again, and the next thing I knew, Papa was in the room, a-standin' over me, hollerin', 'Who done it, who done it?'