There are many examples of the theme of lost innocence in the various plots and sub-plots of To Kill a Mockingbird. Beginning with the early death of their mother, Jem and Scout are left to live out their youth with a different perspective than most children; with the emergence of Dill, another character enters the realm of innocence lost. One of Scout's first experiences comes on her long-awaited first day at school. The teacher, Miss Caroline, scolds and punishes her for various reasons, leaving the budding writer desirous of never returning. A later teacher, Miss Gates, also gives Scout reason to doubt the qualities of Maycomb's classroom instructors.
Both of the children get an up-close view of injustice in action when they witness the virtual entirety of the Tom Robinson rape trial. They both come to the conclusion that Tom is innocent, and they are enraged and saddened by the guilty verdict. The later assault on them by Bob Ewell is another example of an incident that occurs to people much too young. Other examples include the cementing of the knothole by Mr. Radley; the secrets told by Dolphus Raymond; the time spent with Mrs. Dubose and the secret that she holds; and the various experiences of Dill and his absent parents.
The way that Scout is telling the story "as an adult looking back," serves to emphasize the point that she has changed because of the events of the story. As the plot moves, the children move from playing their games without thinking about the adult world to being seriously involved in the adult lives of their neighbors and their adult conflicts in the courtroom and elsewhere.
Certain characters, like Boo, also lose their innocence because of events within the plot. Boo has to emerge from his home to be a part of the outside world, and the town itself even loses some of its semblance of innocence because of the events of the plot.