The beginning of this story is a little iffy and the conclusion leaves one scratching one's head in confusion, but there is quite a bit of entertaining stuff in the middle.
    It begins in Warwick Castle, 1879, during a tour, when the narrator meets a strange gentleman who is too familiar with the armor. Later that evening, he visits the narrator in his room and begins his explanation. His name is Hank Morgan from Connecticut. He was an engineer by trade, specializing in firearms, machinery, engines, and just about anything else that needed to be built. He was so brilliant in his job that he became head superintendent, overseeing a couple thousand rough men. One day, during "a misunderstanding with crowbars" a bruiser called "Hercules" wallops him in the head, and when he awakens he discovers he is not near Hartford, but near Camelot.

(At that point Hank seems unable to stay awake, but invites the narrator to his room, where he hands him a manuscript that details the rest of the story. . .)

    Hank is immediately captured by someone with a large spear (from a circus. . .or asylum probably), so he goes along with him as his "prisoner."
    As they reach the castle, a young page, Clarence, befriends him, and it finally sinks in that June 19, 528 is the present date. . . .Uh-Oh.
    (So he realizes he's in some pretty deep manure, and hasn't a clue how to get out.)
But Hank has several things going for him: he's a brilliant engineer, he's sharp and clever, and HE KNOWS HIS HISTORY. And he knows there's a total eclipse of the sun due in two days. This bit of info comes in handy, because that's the day he will be burned to death. Meanwhile, Clarence becomes Hank's confidant in prison, and goes on and on about Merlin the magician, and his great power. Hank is unimpressed, but figures out a way to outdo Merlin's "magic" (creating a long-term vengeful enemy), to put himself in King Arthur's good standing, to certainly get out of his present mess, and to become such a power across the land that the Catholic Church considers him a threat. Wow! And all this because he knows an eclipse is to occur, and convinces the king HE is causing the darkness. In order for the sun to not be permanently blotted out, Hank does some dealing with the king, making himself "perpetual minister and executive" as darkness overtakes daylight, and by the time the eclipse has ended, "The Boss" is in power.
    The rest of the book is about Hank's adventures in his new era, and how he incorporates nineteenth-century technology into the sixth century.
    Some parts are downright funny, like his description of donning his armor:
Of chain mail, he says, "it is very heavy and is nearly the uncomfortablest material in the world for a nightshirt. . ." He needs help getting it all on as he embarks on an adventure with Alisande, (whom he nicknames "Sandy") and eventually marries. Of course, once his armor is on, he becomes a prisoner inside of it because he can't get it off. The sun beats down and he feels like he's cooking, and he sweats and itches and desperately needs his handkerchief.
    Some of the other stuff ranges from funny to silly, like Hank's merging of nineteenth-century technology with sixth-century life, including education (which was a threat to the Church), newspaper printing, telephones, and winning a joust with a lasso and gun.
    But the part I found disturbing was Hank's obsession with his own power (and assumption that all his modern ideas would naturally make life better), and his preoccupation with killing everyone that he deemed his enemy. (Gosh, does that sound familiar?) Of course, he has explosives, electricity, and firearms, thirteen centuries ahead of the rest. At last, his arch-enemy Merlin puts a curse on him to sleep thirteen centuries. Somehow he gets back to 1879, but we don't know how. Hank really starts to look more nutty than anything else.
    I understand that Twain was writing a satire, and making statements on social injustice, glorified chivalry, and the power of the Catholic Church, but I'm not sure the "time travel" mode was that effective. Still, this one provides some good entertainment and lots of food for thought.