Gripping and complex are the words that best describe To Kill a Mockingbird. The story has several subplots set against the backdrop of racial tension in the 1930s south.
The three story lines that could be described as gripping are the trial of Tom Robinson, the mystery surrounding Boo Radley, and the hatred Bob Ewell has toward Atticus. Each subplot contains rising tension throughout the story and culminates in a satisfying climax.
The complexity of the story is not as much about the plot (and subplots) as it is about the emotions and the "coming of age" character growth exhibited by Jem and Scout. Both children face extreme cases of violence, prejudice and injustice that Harper Lee uses as tools to sculpt the maturation of her characters. Jem and Scout learn valuable lessons about both the good (Atticus) and the bad (racial prejudice) in the world.