Despite his earlier intention of remaining at home, Gulliver returns to sea as the captain of a 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, have turned against him. His crew then commits mutiny. After keeping him contained for some time, they resolve to leave him on the first piece of land they come across, and continue as pirates. He is abandoned in a landing boat and comes upon a race of deformed savage humanoid creatures to which he conceives a violent antipathy. Shortly afterwards, he meets the Houyhnhnms, a race of talking horses. They are the rulers while the deformed creatures that resemble human beings are called Yahoos.
Gulliver becomes a member of a horse's household and comes to both admire and emulate the Houyhnhnms and their way of life, rejecting his fellow 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 commands him to swim back to the land that he came from. Gulliver's "Master," the Houyhnhnm who took him into his household, buys him time to create a canoe to make his departure easier. After another disastrous voyage, he is rescued against his will by a Portuguese ship. He is disgusted to see that Captain Pedro de Mendez, whom he considers a Yahoo, is a wise, courteous, and generous person.
He returns to his home in England, but is unable to reconcile himself to living among "Yahoos" and becomes a recluse, remaining in his house, avoiding his family and his wife, and spending several hours a day speaking with the horses in his stables.