To Infinity and Beyond (or, Time Travel and an Eclipse)
The first act sets up the basic format of the Yankee's adventures, as well as establishing his dilemma: how to survive in the medieval world. We get our first good look at him in action during Chapter 6, when he passes off an eclipse as dark magic that "puts out the sun." This signals the end of the first act because it sets up his position in Camelot, as well as establishing Merlin as his arch nemesis and his clever means of solving problems. We now know everything we need to about his basic situation, and the book's biggest conflicts are set in motion.
Hank is Smarter than the Whole Medieval World (Lather, Rinse, Repeat)
The second act is episodic, consisting of new and interesting ways in which Hank outsmarts… well, everyone. He uses the same basic methods he did with the eclipse time and again: evaluates the situation, applies good old-fashioned know-how to the problems, and masks it all as some kind of magic. It appears repetitive, but we get to see how he evolves and how the rest of society benefits from his smarts in the process. He meets Sandy, he rights wrongs, he builds new and useful inventions that make life less miserable for everyone. As he goes along, however, he makes new and dangerous enemies… who gang up on him in the third act.
Hank's so successful that we take a three-year pause between Act II and III. In many books, that transition marks the point where the hero dusts himself off and takes on the bad guy/killer disease/invading aliens/death asteroid man to man—Twain reverses it here. Hank ends Act II on top of the world, and quickly loses everything as Merlin, the Church, and their various minions come after him. The book ends with Hank back in his own time: heartbroken, dying, and delivering a bitter end to a very sweet life.