When we meet our narrator Huck Finn, he's in Missouri getting "sivilized" ("civilized") by two sisters, an unnamed widow and a woman named Miss Watson. See, Huck Finn came into a bit of money at the end of Tom Sawyer, and now he's supposed to stop being a street urchin and start learning to be a gentleman. But it's hard out there for a street urchin, and he spends most of his time avoiding baths and teaming up with Tom to punk innocent bystanders—like Miss Watson's slave Jim.
When Huck's spidey sense starts a'tingling, he signs over all his money to Judge Thatcher. Just in time: Huck's deadbeat dad shows up and demands the money. Huck's all, "too bad you didn't get here yesterday, dad," and then dad effectively kidnaps Huck and takes him off to live in filthy poverty in a van down by the river. (Only without the "van" part.)
Well, Huck isn't too cool with this, so he (naturally) fakes his own death and hides out on a nearby island, where he meets another runaway: the slave Jim, who's hiding out to avoid being sold down South and separated from his family. After running across a dead body, which Huck doesn't see, they decide to team up and then start out on what just might be the first American road movie, only via the Mississippi River rather than I-90.
Cue a series of wacky hijinks/ life-threatening situations, like:
Meanwhile, Huck keeps wrestling with his conscience: is he helping an innocent man escape slavery, or is he just stealing Miss Watson's property? He decides that helping Jim escape is the right thing to do—even if he goes to "hell" for it—but, unfortunately for Jim, it's not up to Huck. Jim is recaptured, and things quickly go south. Pun intended.
Eventually, Tom shows up and teams with Huck to help Jim escape a hut where he's being held captive. Their elaborate plan goes awry. Tom is shot, and Huck falls asleep while waiting for a doctor. When he wakes up, the situation is out of his hands. Jim is about to be executed, when Tom announces that (1) Jim saved his live, and (2) Miss Watson actually freed Jim in her will when she died two months ago. Hooray!
And to wrap things up just a little more neatly, it turns out that the dead guy from the island was Huck's dad, so that loose plot point is all tied up; plus, Huck still has all the money he found at the end of Tom Sawyer. Time to stick around and get a little of that civilizing he keeps talking about?
Not quite. Instead, Huck heads out west, ready for more adventures.