Although neither Tom Robinson nor Bob Ewell (the other central figures of the primary plot of Part Two) were mentioned in the opening chapter, the author quickly established the importance of Boo Radley in the lives of Jem and Scout. Most of the chapter discusses the history of Boo and the Radley family, and when Dill is introduced, his own curiosity of Boo further instill's the Finch children's interest. Miss Stephanie's insistence that Boo did come out--only at night--only peaked the children's curiosity further. When Dill's proposal,
"Let's try to make him come out..."
is made, it becomes the children's main goal for the next year. The fact that Boo is capable of murder is also established, though it is merely speculation from the imaginations of children.
Jem said if Dill wanted to get himself killed, all he had to do was go up and knock on the front door.
"... I hope you got it through your head that he'll kill us, each and every one, Dill Harris," said Jem.
Jem later prophetically remarked to Scout,
"If I got killed, what'd become of you?"
The fact that Boo is never seen is important, since the reader must assume that a character of such impact must eventually make an appearance in the story. And Boo almost does at the end of the chapter.
... we thought we saw an inside flutter move. Flick. A tiny, almost invisible movement...