Chapter 19: In this chapter, after Huck and Jim have been traveling for a bit, they encounter two men who are running for their lives, being chased by dogs and men, they say. They ask Huck if they can get away with them, and Huck agrees. Eventually the men announce to each other and to Huck and Jim that they are actually a king and duke, who have lost their kingdoms. Huck doesn't say anything, but he admits to the reader that he thinks "these liars warn't no kings nor dukes, at all, but just low-down humbugs and frauds."
Chapter 20: Wondering about Jim, the king and duke ask Huck if he's a runaway slave. Huck is adamant in his denial, saying that they wouldn't be traveling south if he were a fugitive slave (They are forced to travel south now because of the current, which of course heads south on the Mississippi.). Huck follows his denial by telling them a compelling story about themselves. Here and elsewhere in the book, it becomes apparent to the reader that Huck is an amazing liar.
It also soon becomes apparent what the king and duke do for a living: they are frauds and cheats, just as Huck suspected. Stopping at a town downstream, both the king and the duke leave the raft, using their knack for deception to cheat quite a sum of money out of the people.
Chapter 21: Now in Arkansas, the king and duke begin orchestrating their next scheme: the enactment of the balcony scene from Romeo and Juliet. Also in this chapter Huck witnesses a mob scene caused by a petty local dispute. Huck notices that the people in the mob act on the courage of others, instead of taking individual initiative of their own.