Tom's summer is so exciting that he joins the "Cadets of Temperance," which is sort of like the Boy Scouts, we guess, but instead of doing cool stuff in the woods, they march around and denounce the devil's drink, alcohol. This kind of stuff was popular back then.
Tom is mostly attracted by the fancy uniform he gets to wear, but he doesn't have any occasion to try it out; he's so desperate to don it that he hopes the old judge will kick the bucket so he can march in his funeral procession. Tom is a little too impatient, though, and the judge dies just after he hands in his resignation.
Tom attempts to keep himself occupied by writing a diary, attending a traveling minstrel show, going to the circus, seeing various other traveling acts, and partying hard with his crew. None of this helps.
Bored out of his skull, Tom begins to think about the murder again…and then he gets the measles. All is not well in St. Petersburg.
When he finally gets better, he finds that Joe and his other buddies have "found religion"; and even Huck has taken to quoting Scripture.
After one night of terrible, boring freedom, Tom relapses and spends another three weeks in bed with the measles. By the time he regains his strength, things have, praise the Lord, returned to normal. He finds Huck and Joe eating a stolen melon.