Two astronauts survive a crash-landing on a planet where intelligent apes rule over the inferior humans. The men find that their own intelligence challenges the apes' dogma - and puts their lives in danger.
When they are buried alive, underground, in an ancient San Francisco subway station during an earthquake, Burke and General Urko are forced to work together. Above ground, Galen and Verdon try to figure out a way to help Burke, with the assistance of several gorilla soldiers.
When Galen hurts his leg and can't travel, Virdon and Burke seek refuge in a farm till their friend recovers. The farm is operated by a gruff but decent ape named Polar, his wife Zantes, and their three children: older son Anto, younger son Remus, and daughter Jillia. Polar puts to work the two strangers who teach him about contour farming, windmill building, seed selection, etc. Polar soon accepts these changes but Anto resents the strangers, fearing they will hex a cow about to give birth. (Until the cow delivers a bull calf, Anto can't start a farm of his own.) ...
While walking on an ocean beach with Galen, Virdon and Burke see a condemned man lashed to a raft in the water. They rescue him and carry him to a nearby cave but then an ape-on-patrol captures the two astronauts and takes them to his boss, Hurton. Hurton administers a fishing operation and he presses Virdon and Burke into service as fishermen after they pass a test by swimming under a sheet of flames. Trouble arises when Hurton's superior, Bandor, comes on an inspection tour, but the astronauts save the day by demonstrating how a net can catch more fish than spears. ...
Galen risks his freedom, and life by contacting his old sweetheart, Kira, when Virdon is seriously injured. Kira is now a talented, and forward thinking, surgeon; and Galen shares with her a forbidden book on human anatomy, that he and Burke stole from Dr. Zaius' home, with her.
Galen, Verdon and Burke befriend, Fauna, the blind ape daughter of a human friendly ape who was killed by a murderous band of vigilante apes, calling themselves 'dragoons.' Unaware that Burke is human, Fauna falls in love with him, as Galen and Virdon hunt down the 'dragoons', only to discover that fauna's own uncle is one of them.
Virdon agrees to race a chimpanzee Prefect's horse against Urkos best horse, in exchange for a condemned boy's freedom. Urko has never lost a race, but of course, as Virdon finds out, Urko doesn't play fair. Despite Urko's treachery and cheating, Virdon prevails in the end.
When Burke is captured by Urko's soldiers, he is forced to face two equally horrifying options: he can either be killed, outright, by Urko or he can be used in a brainwashing experiment, being performed by a prominent ape doctor, using a forbidden (human) text on the subject. The scene in which Urko describes what he believes brainwashing is, is hilarious.
In an attempt to foil the plans of a tyrannical gorilla lieutenant, who is using bribery to replace the chimpanzee Prefect, and gain complete control over a human farming district; Galen, Virdon and Burke risk an encounter with Urke, forcing Urko and Burke to work together, once again (previously in The Trap - Air date: 9/27/74).
Virdon and Burke race to concoct a cure, to save villagers stricken with malaria, with the aid of an ape doctor. They must do this before Urko implements his own version of a cure, by burning down the entire village with everyone in it.
Virdon and Burke's only hope for survival is to convince, with Galen's help, their human captors to rebel against the apes, with whom this tribe of humans turns their fellow humans over to as slaves. This episode was not aired during the series original run.
Unaware that a chimpanzee scientist plans to use the flying machine of an imprisoned man, whose life she spared, for her own deadly purpose; Virdon, Burke and Galen help the man to build a hang glider. This was the final episode of the series.