Gulliver returns to sea as the captain of a 35 ton merchantman as he is bored with his employment as a surgeon. On this voyage he is forced to find new additions to his crew who he believes to have turned the rest of the crew against him. The pirates then mutiny and after keeping him contained for some time resolve to leave him on the first piece of land they come across and continue as pirates.

Gulliver is  is abandoned in a landing boat and comes first upon a race of  hideous deformed creatures to which he conceives a violent antipathy. Shortly thereafter he meets a horse and comes to understand that the horses (in their language Houyhnhnm or "the perfection of nature") are the rulers and the deformed creatures ("Yahoos") are human beings in their base form. Gulliver becomes a member of the horse's household, and comes to both admire and emulate the Houyhnhnms and their lifestyle, rejecting humans as merely Yahoos endowed with some semblance of reason which they only use to exacerbate and add to the vices Nature gave them.

 However, an Assembly of the Houyhnhnms rules that Gulliver, a Yahoo with some semblance of reason, is a danger to their civilization and he is expelled. He is then rescued, against his will, by a Portuguese ship, and is surprised to see that Captain Pedro de Mendez, a Yahoo, is a wise, courteous and generous person. He returns to his home in England, but he is unable to reconcile himself to living among Yahoos and becomes a recluse, remaining in his house, largely avoiding his family and his wife, and spending several hours a day speaking with the horses in his stables.