Tom Robinson, a black field worker, is accused of raping and beating Mayella Ewell. In a town like Maycomb, if a black man is accused of doing anything to a white woman, then that means he is guilty, no matter what. Just about everyone in the town thinks that Tom is guilty, and Atticus is doing his best to prove that Tom is innocent. We learn that Mayella was beaten on the face and strangled. She had two hand prints around her neck, so we know that whoever did this was strong and had the use of both of his hands, and also was left-handed. When Atticus asks Tom to stand up so Mayella can get a good look at him, we discover that there is no way Tom is guilty.

Tom Robinson's powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him.
"Scout," breathed Jem. "Scout, look! Reverend, he's crippled!"
Reverend Sykes leaned across me and whispered to Jem. "He got it caught in a cotton gin, caught it in Mr. Dolphus Raymond's cotton gin when he was a boy...like to bled to death...tore all the muscles loose from his bones-"

Atticus then asks Bob Ewell to write his name on a piece of paper, and it is shown that Bob is left handed. We learn that Tom in no way could have been the one to commit the crime against Mayella, and we learn that Bob is more than capable of doing such a horrible act. We are about to learn just how far Bob is willing to go to get what he wants, and just how dangerous a man he can be.