The novel is a comedy set in 6th-century England and its medieval culture through Hank Morgan's view; he is a 19th-century resident of Hartford, Connecticut, who, after a blow to the head, awakens to find himself inexplicably transported back in time to early medieval England where he meets King Arthur himself. Hank, who had an image of that time that had been colored over the years by romantic myths, takes on the task of analyzing the problems and sharing his knowledge from 1300 years in the future to try to modernize, Americanize, and improve the lives of the people.

Many passages are quoted directly from Sir Thomas Malory's Le Morte d'Arthur, a late medieval collection of Arthurian legends that constitutes one of the main sources on the myth of King Arthur and Camelot. The frame narrator is a 19th-century man (ostensibly Mark Twain himself) who meets Hank Morgan in modern times and begins reading Hank's book in the museum in which they both meet. Later, characters in the story retell parts of it in Malory's original language. A chapter on medieval hermits also draws from the work of William Edward Hartpole Lecky.

Introduction to the "stranger"

"'Bridgeport?' said I, pointing. 'Camelot', said he."

The story begins as a first-person narrative in Warwick Castle, where a man details his recollection of a tale told to him by an "interested stranger" who is personified as a knight through his simple language and familiarity with ancient armor.

After a brief tale of Sir Lancelot of Camelot and his role in slaying two giants from the third-person narrative, taken directly from Le Morte d'Arthur, the man named Hank Morgan enters and, after being given whiskey by the narrator, he is persuaded to reveal more of his story. Described through first-person narrative as a man familiar with the firearms and machinery trade, Hank is a man who had reached the level of superintendent because of his proficiency in firearms manufacturing, with 2000 subordinates. He describes the beginning of his tale by illustrating details of a disagreement with his subordinates during which he sustained a head injury from a "crusher" to the head caused by a man named "Hercules" using a crowbar.

After passing out from the blow, Hank describes waking up underneath an oak tree in a rural area of Camelot, where he soon encounters the knight Sir Kay, riding by. Kay challenges him to a joust, which is quickly lost by the unweaponed, unarmored Hank as he scuttles up a tree. Kay captures Hank and leads him towards Camelot Castle. Upon recognizing that he has time-traveled to the 6th century, Hank realizes that he is the de facto smartest person on Earth, and with his knowledge he should soon be running things.