Gulliver meets the Houyhnhnms, horse-men who rule, and Yahoos, brutish humanlike creatures who serve the Houyhnhnms. Gulliver sets about learning their language, he narrates his voyages to them and explains the constitution of England. He is treated with great courtesy and kindness by the Houyhnhnm.

Finally, on his fourth journey, Gulliver sets out as captain of a ship, but after the mutiny of his crew and a long confinement in his cabin, he arrives in an unknown land. This land is populated by Houyhnhnms, rational-thinking horses who rule, and by Yahoos, brutish humanlike creatures who serve the Houyhnhnms.

Gulliver sets about learning their language, and when he can speak he narrates his voyages to them and explains the constitution of England. He is treated with great courtesy and kindness by the horses and is enlightened by his many conversations with them and by his exposure to their noble culture.

Gulliver wants to stay with the Houyhnhnms, but his bared body reveals to the horses that he is very much like a Yahoo, and he is banished.

Gulliver is grief-stricken but agrees to leave. He fashions a canoe and makes his way to a nearby island, where he is picked up by a Portuguese ship captain who treats him well, though Gulliver cannot help now seeing the captain—and all humans—as shamefully Yahoolike. Gulliver then concludes his narrative with a claim that the lands he has visited belong by rights to England, as her colonies, even though he questions the whole idea of colonialism.