A narrator identified as “M.T.” (pointing to the book’s author, Mark Twain) encounters a strange tourist (Hank Morgan) at Warwick Castle in England. It turns out that both men are at the same hotel, and later that night, Hank begins to tell M.T. his life story. He was born and raised in 19th-century Hartford, Connecticut. Hank blacked out after receiving a blow to the head during a workplace brawl, though—and when he woke up, he was in medieval England. M.T. becomes too tired to go on, and so he gives M.T. a book containing his life story to read.
Hank’s book begins with a knight named Sir Kay capturing Hank and bringing him to Camelot as his prisoner. In Camelot, Hank manages to escape execution and establish a reputation as a powerful magician by predicting a total solar eclipse. Afterward, he uses his 19th-century knowledge to blow up resident sorcerer Merlin’s tower. (Hank creates blasting powder, places it in the tower, and connects it to a lightning rod. Then, he uses his talent for showmanship to work the miracle during the next thunderstorm, utterly convincing the primitive medieval people that he has the power to control nature itself.)
Hank’s first two miracles vault him to the second-most powerful position in the kingdom after King Arthur, earning him the title “The Boss.” Hank is busy laying the groundwork for an educational, social, and political revolution when Sir Sagramore challenges him to a duel. Fortunately, the date is set three or four years into the future so that Sagramore can go on a quest for the holy grail. Toward the end of this period, Hank is assigned a quest of his own when a young woman named Sandy arrives at Camelot with a horrific tale of being imprisoned (along with dozens of other ladies and princesses) by a trio of ogres. On their way to rescue the ladies, she and Hank have some other minor adventures. They stay the night with Morgan le Fay, whose brutality both horrifies and impresses Hank. When they reach Sandy’s “ladies,” Hank is shocked to find that the women are really a herd of pigs, and her “ogres” are a trio of scrawny swineherds. But Sandy is convinced that they’ve been enchanted to look like animals to Hank, and Hank “rescues” them to humor her, thus completing his quest.
No sooner have Hank and Sandy turned back toward Camelot than they encounter a group of pilgrims who are traveling to see a miraculous fountain in the Valley of Holiness. When the bad news arrives that the fountain has dried up, Hank uses 19th-century technology to restore it, adding another “miracle” to his repertoire.
Following the success of his quest and his growing list of miracles, Hank decides to see how the common people live by disguising himself as a freeman and traveling incognito around the kingdom. King Arthur, delighted with the idea, insists on joining Hank. During their travels, King Arthur and Hank learn how difficult life is for the common people, who are denied mercy and common humanity by the knights, their lords, or church authorities, die of smallpox, and can barely support families thanks to heavy tax burdens.
But when Hank, ever the showman, overreaches and offends a group of freemen in a small village, he and the Arthur find themselves running for their lives. A gentleman rescues Hank and Arthur from a mob of angry villagers—only to sell them into slavery. sells them into slavery.
Hank and Arthur witness more scenes of brutality as they’re taken to London to be sold at auction. In London, Hank escapes and instigates a slave uprising that kills the slave master, for which all the slaves are condemned to death. Hank is recaptured and taken to the gallows with the rest of the slaves, but Sir Launcelot and a rescue party of 500 knights rides into the city on newfangled bikes and rescue them just in the nick of time.
Restored to his position of authority in the kingdom, Hank prepares for his duel with Sir Sagramore. In the years since his arrival, he’s quietly been laying the groundwork for a civilizing revolution that will bring 19th-century technology, moral sensibilities, and democracy into the sixth century. Only the chivalric code of knights and the Church stand in Hank’s way, and the duel with Sagramore is his chance to show his superiority to chivalry once and for all. Although Merlin has allegedly enchanted Sagramore’s armor to protect him, Hank kills the man with one shot from his revolver before dispatching nearly a dozen other knights in the same way. Having utterly humiliated and defeated knighthood, Hank is free to go public with his secret plans to reform sixth-century society.
Three years later, Hank sits at the head of a humming 19th-century economy. He’s now married to Sandy, and they have a daughter named Hello-Central. Hank is working on the final part of his plan: getting King Arthur to make a decree dissolving the monarchy upon his death. But then Hello-Central falls sick, and Hank and Sandy take her to recover by the sea in France. While they’re gone, a civil war breaks out between King Arthur and Launcelot, and Arthur’s nephew Mordred seizes the throne. Arthur and Mordred kill each other in battle, the Church places the island under interdict, and Hank’s 19th-century innovations come to a screeching halt.
Hank returns to England to find his second-hand man, Clarence, and 50 young boys ready to fight with him on the side of 19th century; 30,000 men ride against them. Hank and his boys defeat the knights with electric fences and explosives. But in doing so, they trap themselves behind a wall of dead bodies. The decomposition of these corpses causes disease, Hank falls into a coma, and the rest of the boys die, too.
M.T. finishes reading the manuscript and goes to the stranger’s room. The door is ajar, and when he pushes it open, he sees the stranger lying in bed, delirious. M.T. stays and listens to his feverish ravings until the man’s strength fails, and he dies.