As the astronauts begin to travel away from the crash site through the desert, they eventually discover some plant life. While swimming in a pool they discovered in the desert, their clothes are stolen by some primitive human natives. While chasing the thieves, they are captured by a group of intelligent gorillas and separated.
The one astronaut named Taylor is placed in a cage with some other humans to be studied by Chimpanzee scientists. These scientists are looking for the missing link between humans and intelligent apes. During capture Taylor's throat is hurt, causing him to lose his voice, but they believe him to be an intelligent human and want to study him further.
He eventually escapes and is recaptured in time for him to speak. Treated like an animal, he is brought before a council where he learns that his colleagues have been experimented on with brain surgery and turned into unintelligent mutes. The council decides that Taylor should be taken for surgery as well or put to death to hide his intelligence.
Chimpanzee doctors Cornelius and Zira manage to help Taylor escape again. On the run, they discover a cave. After examining the contents of the ancient civilization that lived there, Taylor discovers that he is not on a different planet but that this is earth of the future.