The sole survivor of an interplanetary rescue mission discovers a planet ruled by apes and an underground city run by telekinetic humans.

Taylor, an astronaut, has crashlanded on an unknown planet. Having escaped from his captors--uniformed gorillas on horseback--Taylor, accompanied by his mute female companion Nova, is riding on horseback over a wasteland, an area feared by the apes as the "forbidden zone," when he encounters the remnants of the Statue of Liberty, half buried in the sand, and he realizes that his 20th-century world has been destroyed. Suddenly the earth splits before them, and Taylor vanishes into the rock formation after telling Nova to return to Zira, a scientist and one of the friendly apes who helped them escape. En route Nova meets another astronaut, Brent, who has been sent to find Taylor, and takes him to Taylor's friends in the city of the apes. They are captured after learning that the militaristic apes plan to destroy the remaining life in the forbidden zone. Zira helps Brent and Nova escape into the forbidden zone, and there they stumble onto the subterranean ruins of New York City. The labyrinth is populated by human beings, 40th-century mutations who conceal their disfigurations with masks, worship a live atom bomb, and communicate telepathically. Brent is captured, interrogated about the planned ape attack, and thrown into a cell with Taylor. Their guard orders them by telepathy to kill each other, but Nova interrupts the guard's thought, and in the ensuing respite, Taylor and Brent overpower the guard. Meanwhile, the apes attack the city, in spite of the mutants' attempts to repel them. Brent and Taylor try to prevent the detonation of the atom bomb, which they realize is a doomsday machine capable of destroying the world. Nova is killed, along with the mutants' leader. Taylor is shot; Brent is killed; and, seeing that the apes can easily win, Taylor presses the button which releases the bomb.