What is the main plot for To Kill A Mockingbird?

I am not sure I understand your question, but the main plot in To Kill A Mockingbird concerns Scout's coming of Age as she grows up in the racially segregated south.  The major event which leads to the climax of the story is the trial of Tom Robinson, a black man, for the attempted rape of Mayella Ewell.  Although Tom is clearly innocent of the charges, he is found guilty because people in the south were not ready to take the word of a black person over that of a white person no matter what the circumstances.  Scout's father, Atticus, who is the moral icon of the story, defends Tom to the fullest of his abilities despite knowing the futility of his effort.  He does so because it is the right thing to do.  For some the trial itself is the climax of the story, but I believe the climax of the story is the attack on Jem and Scout because this is the final point of growth for Scout and resolves the subplot concerning Boo Radley.