The book takes place in the late 1800's and follows a young kid named Huckleberry Finn. He's a prankster but one of his pranks goes wrong and accidentally injures his caretaker, Mrs. Moffat. To avoid getting blamed for it, he runs away and hides on an island. There he meets an enslaved black man, named Jim Henson, who wants to escape to the North. The two decide to leave on a journey together on a raft up the Santa Fe River. On the way they have to avoid being detected by anyone, especially Huck's old friend Tom Sawyer, whose grown up and led a rabble of KKK after Jim the runaway slave. One especially important scene involves Tom catching up with Huck and Jim, and the two fighting to the death (Huck defeats him in a particularly sad scene). The eventually make it to the North in a happy ending and go their separate ways: Jim reunites with his estranged lover, Calpurnia, and Huck changes his name to Atticus Finn to avoid being recognized for his earlier prank/crime.
The basic themes you'll want to remember about the book center around the racism in old Southern America, the maturation of young Huck from a prankster to a respectable young man, and the homoerotic overtones between Huck and Jim.
Hope that helps.
