Some time after 3200 c.e., Jinn and Phyllis are on a recreational ride in their specially designed spacecraft when they notice a bottle drifting near them. Inside it they find a long message written in “the language of the Earth.” They are startled to find that it repeatedly refers to humans as intelligent beings. Ulysse Mérou, a young French journalist, wrote the message, which relates the story of his experiences.

In 2500, Ulysse accompanies the wealthy Professor Antelle and Levain on a privately funded expedition to a planet of the star Betelguese. The passengers on Antelle’s spaceship will experience the round-trip journey as taking two years in each direction. However—because of the relativistic effects of stopping at the destination planet and turning around to come home—when they return, about seven hundred years will have passed on Earth.

The travelers reach their destination safely. The atmosphere seems breathable. Exiting their craft, the men soon see a small human footprint on the shore of a lake. A gorgeous, naked young woman appears and dives into the water. The explorers name her Nova. She fears the men until they remove their clothes. Then, she frolics near them but seems unable to speak, and she kills their pet chimpanzee.

A band of wild humans peers furtively from the underbrush. Finding the earthmen’s clothes on the bank, they tear them into shreds and damage the explorers’ landing craft beyond repair. The savages do not harm the explorers themselves, but soon a hunting party of clothed, talking, civilized apes appears. The apes wantonly shoot many of the humans and capture others, including the explorers. The survivors are taken to zoos or to research labs.

Ulysse is placed in the same cage as Nova and eventually mates with her. He displays such intelligence that the lab director Zira is platonically drawn to him. They gradually learn each other’s language, and Zira comes to believe his claim to have arrived from another planet. On her planet, humans are bestial. Zaius, however, insists that Ulysse’s apparent cleverness derives from tricks learned by rote during an earlier period of captivity. When Ulysse addresses Zaius and Zira by name, Zaius insists that Ulysse is merely parroting what he hears.

Ulysse learns that each of the three advanced species on the Planet of the Apes has separate talents and functions. The orangutans rule “official science” as figureheads. They are pedantic, stuffy, narrow-minded, and obsessed with honors and distinctions. The chimpanzees are brilliant, sensitive, and creative. The gorillas perform jobs requiring brute strength, such as being hunters and guards, but they also serve as organizers, office managers, and businessmen.

Zira obtains permission to take Ulysse out into the city, but he must go naked and wear a muzzle, leash, and chain. He is impatient to be free, but Zira warns him that Zaius is extremely powerful and wishes to send Ulysse to the brain lab, where experimental operations would transform him into a cripple or a vegetable. Ulysse’s only hope is to unmask his rational nature in a formal speech, in ape language, when the national biological congress meets in a month. Members of the public and many journalists will attend, and the power of public opinion alone can defeat Zaius.

Zira helps Ulysse prepare by slipping him a flashlight and several books in ape language, which he conceals in his cage. On a trip to the zoo, Ulysse encounters Antelle, who has been reduced to a subhuman state by his captivity and is unable to speak or recognize his former friend. At the congress, Ulysse delivers a humble, impassioned, persuasive appeal for interspecies cooperation. Afterward, he faints from the strain but is freed, clothed, and accepted into the ape community.

Now a lab assistant, Ulysse tries in vain to teach captive wild humans to talk. Cornelius takes him to an archaeological excavation, which shows evidence that a civilization as advanced as the ape’s own once existed there more than ten thousand years earlier. A doll in human form, which is clothed and which talks, suggests that the ancient civilization may have been human.

Ulysse learns that Nova has become pregnant. If the baby seems to have intelligence superior to that of an animal, it and Ulysse will be seen by the apes as highly dangerous: Such intelligence in his offspring would be an indication that it is possible for Ulysse to breed a human super-race to challenge ape supremacy. Father and child would surely be killed. Meanwhile, Ulysse is taken to see gruesome experiments in the brain surgery lab. One female subject, however, has recovered ancestral memories. She recites them in a trance, telling how humans gradually degenerated until their ape slaves took over and the humans fled into the jungle.

Nova gives birth. Because all humans look alike to the other apes, Zira and Cornelius arrange to save Ulysse and his family by secretly substituting them for other human experimental subjects who are to be sent aloft in a space shuttle. Cornelius’s friends have detected the location of Ulysse’s mother ship and programmed the shuttle to rendezvous with it. Zira is strongly moved at Ulysse’s departure, but cannot bring herself to kiss him good-bye because she finds him so unattractive.

The escape succeeds. On the voyage home, Nova learns to talk, and her child Sirius displays extraordinary intelligence. However, when the family reaches Paris and lands at Orly airport, the officer who meets their craft is a gorilla. Evidently, humans on Earth have degenerated also, as they did on the Planet of the Apes, and Ulysse must again flee into outer space.

Jinn and Phyllis finish reading Ulysse’s narrative. Jinn persuades Phyllis that the idea of intelligent humans is too absurd to take seriously. As the couple returns to their home planet, Phyllis takes out her compact and powders “her dear little chimpanzee muzzle.” Capable only of imitating human inventions on the Planet of the Apes at the time of Ulysse’s visit there, the chimpanzees have now become capable of creating inventions of their own, and apes may have conquered the entire galaxy.