Huck and Jim run away from their respective enslavements, and we're ready for a wacky road movie. For a while, we get it: they hide out in a cave, trick some robbers, and have a series of nail-biting but not life-threatening adventures. And then they get separated—which means it's time for Act II.
During Act II, Huck continues to have misadventures. Some are serious, like the deadly feud between the Grangerfords and the Shepherdsons. Some are not so serious, like the hilariously incompetent conmen. But while Huck is off having the time of his life, Jim is tied up, hiding out, and generally (we imagine) taking the whole thing a lot more seriously. Huck may be on a lark, but Jim? He's got his life, liberty, and family to look after.
Act III begins when Jim is re-captured. Things get serious for a little while, but they quickly become comic when that rascal Tom shows up with his kooky ideas about sawing Jim's leg off. Hilarious! We're rolling in the aisle. Right?
Well, maybe. In any case, things quickly wrap up: Tom is shot, but not seriously; Jim is almost hanged, but is freed in the end. And Huck? He's back off to the open road.