In a captivating frame story, we encounter a wealthy couple, Jinn and Phyllis, who are journeying through the vastness of space. During their travels, they stumble upon a mysterious manuscript carefully preserved inside a floating bottle. This manuscript, penned by the adventurous Ulysse Mérou, a French journalist, recounts his extraordinary experiences from the year 2500. Ulysse had been invited by the enigmatic Professor Antelle, accompanied by his eager disciple, the physician Arthur Levain, to embark on a thrilling expedition to the distant star, Betelgeuse.

As they traverse the cosmic expanse, time dilation causes centuries to slip away on Earth during their brief two-year voyage. Eventually, the trio lands on a vibrant, lushly forested planet, which they aptly name Soror—Latin for sister. Soon after their arrival, they encounter a stunningly beautiful naked woman who they affectionately label Nova. In a playful scene, the trio finds themselves swimming joyously beneath a majestic waterfall; however, their pet chimpanzee, Hector, inadvertently frightens Nova, leading to his untimely demise at her hands.

The story takes an unexpected turn when a group of gorillas, masquerading as big-game hunters, launch a ruthless attack on Nova’s tribe, resulting in a devastating conflict. Tragically, during this chaotic event, Arthur is killed, and Ulysse finds himself captured and transported to a metropolis inhabited by intelligent apes. Much to his astonishment, these apes partake in various civilized activities: they smoke tobacco, photograph their hunting trophies, and sip drinks through straws, showcasing a complex social structure. This society is starkly divided into three distinct classes: the militaristic gorillas act as enforcers and lawmen, the conservative orangutans serve as politicians and religious leaders, while the progressive chimpanzees fulfill roles as scientists and intellectuals.

Within an urban biological research facility, Ulysse soon discovers alarming conditioning tactics being conducted on captured humans, leading to his own forced mating with Nova. Here, Zira, a curious chimpanzee researcher, becomes fascinated by Ulysse’s geometric drawings and his rudimentary mastery of their simian language. With the assistance of Zira’s fiancé, Cornélius, Ulysse ingeniously manages to deliver a momentous speech in front of an audience of several thousand apex creatures. This act of bravery earns him freedom, social prestige, and even stylish, tailored garments, starkly contrasting the fate of Professor Antelle, who regresses to primitive behavior and is confined to the lab for his own protection.

Cornélius, an avid archaeologist, invites Ulysse to explore the remnants of an ancient human city, where they uncover staggering revelations from the genetic memory of a comatose human experiment. The haunting narrative paints a grim picture of humanity: once dominant, humans began to tame apes and utilize them as subservient helpers, but as the apes developed speech, the humans experienced a debilitating intellectual decline. With apes gradually overtaking human households and banishing their former masters to camps outside the urban sprawl, the final chilling memory recounted is a brutal assault on the last vestige of human existence.

Amidst these striking developments, Nova gives birth to Ulysse’s son, whom they name Sirius, who astonishingly begins to walk and talk by just three months old. Fearing for their newfound family’s safety, Ulysse and Nova decide to substitute themselves for human test subjects in a space flight experiment, counting on the apes’ inability to discern human faces. In an escape fraught with tension, they successfully rendezvous with their orbiting ship and set their course back to Earth.

As they soar above Paris, familiar landmarks like Orly Airport and the Eiffel Tower emerge beneath them, rekindling their hopes. However, upon landing, they are met with a chilling reality: a gorilla field officer awaits them in a jeep. Overcome with horror at their predicament, Ulysse and Nova make a frantic escape in their spacecraft, permanently leaving Earth behind. In a moment of reflection, Ulysse commits his memories to writing, crafting the manuscript and casting it into space as a testament to the unbelievable journey they endured.

As the frame story unfolds, we learn that the wealthy couple, Jinn and Phyllis, are not what they seem; they are, in fact, chimpanzees themselves. Discarding Ulysse’s tale as mere fantasy, they struggle to accept the possibility of intelligent humans having existed, highlighting a poignant irony that lingers long after the story concludes.