Because they travel close to the speed of light, time dilation causes centuries to pass on Earth during their two years in transit. They land their shuttle on a temperate, lushly forested planet which they name Soror (Latin for sister). They can breathe the air, drink the water and eat the fruit. Attracted by a lovely golden naked woman whom they call Nova, they swim below a scenic waterfall. She is frightened by their pet chimpanzee, Hector, and strangles it. Her tribe, who comport as dumb animals, wreck the newcomers' clothing and shuttle.
Gorillas, fully dressed as hunters, attack the tribe with firearms. Many are killed, including Arthur. Ulysse is captured with the survivors and brought to an entire city populated by apes. Ape clothing matches that of 20th century Earth humans, except that the apes wear gloves instead of shoes on their prehensile feet. The apes smoke tobacco, photograph their hunting trophies, drink through straws and appear utterly civilized. Their society is divided into three strata: aggressive gorilla police and military, conservative orangutan politicians and religious authorities, and liberal chimpanzee scientists.
In an urban biological research facility, Ulysse recognizes Pavlov's dog conditioning being used on captured humans. He is mated with Nova. Curious chimpanzee researcher Zira takes an interest in his geometric drawings and his ability to speak a few simian words. With help from her fiancé, Cornélius, Ulysse makes a speech in front of several thousand apes. He is granted freedom and is given tailored clothing. Antelle reverts to primitive humanity in the zoo and is moved to the laboratory for safety, where he is mated to a young female.
Cornélius, an archaeologist, excavates an ancient human city. An unconscious human lab subject recites from racial memory the events that led to the fall of human civilization: humans tamed apes and eventually used them as servants. Things began to change. As apes learned to talk, a cerebral laziness took hold of the humans. Apes gradually took over human homes, driving the humans into camps outside of the cities. In the final memory, apes attacked the last human camp, carrying only whips.
Nova bears Ulysse a son, Sirius, who walks and talks at three months. Fearing for their lives, they take the place of the human test subjects in a space flight experiment. Because all humans look alike to apes, they are able to escape without notice and they rendezvous with the orbiting ship.
Ulysse programs the ship back to Earth. As they fly over Paris, Orly Airport and the Eiffel Tower look the same. When they land, however, they are greeted by a field officer in a Jeep who is a gorilla.
It is subsequently revealed, via the frame story, that Jinn and Phyllis are actually civilized chimpanzees, and they discard Ulysse's story as sheer fantasy because the idea of humans as speaking and civilized members of society is an utterly unbelievable concept to them.