What is the story of Huckleberry Finn?

What's the essence of the narrative journey undertaken by Huckleberry Finn in Mark Twain's classic novel? How does it capture the essence of American frontier life and challenge societal norms through its portrayal of Huck's coming-of-age amidst the backdrop of slavery and the Mississippi River? How does Huck's character evolution reflect the complexities of human morality and the struggle between individual freedom and societal constraints?