In the silent void of outer space, astronaut Bill Hudson transmits to Earth from the NASA spacecraft Venturer. On board are crew members Jeff Carter and Judy Franklin. The ship's date is August 6, 1976. Bill explains in his transmission that they are living proof of Dr. Stanton's "time thrust" theory. Due to the advanced speed of the Venturer, their Earth clock indicates that they have traveled over a century into the future. Without warning, the craft begins to reel wildly out of control as the Earth clock starts clicking up. The crew blacks out while the ship enters a planet's atmosphere and crashes into a desolate desert region. Meanwhile, General Urko, leader of the gorilla army, is debating before the Supreme Council of Ape City. He demands permission to begin an all-out assault on the humanoids, ending in their annihilation. Cornelius, an animal psychologist, pleads for the council to allow the humanoids to live as subjects of scientific research. He believes the knowledge gained will help the simian culture better understand their origins. Urko violently disagrees and the crowd supports him. After hearing both sides presented, the Council closes the proceedings by decreeing that the humans should not be exterminated since they have no language, but they will continue to be hunted, enslaved, domesticated and studied. According to the Book of Simian Prophecy, however, the humanoids must be destroyed if it is ever discovered that they can speak.
By now, Bill, Jeff and Judy have escaped their disabled, sinking ship and rafted to shore. Bill's watch reveals the date still to be August 6, but in the year 3979 A.D. With no better plan, they begin trekking across the wasteland, hoping to find some kind of life to help them. They encounter sudden electrical storms, high-velocity winds, tremors, unexplained flames and ruins proving that some kind of intelligent life once inhabited the region. Exhausted and at the end of her strength, Judy pleads with Bill and Jeff to go on without her, but they refuse. Another mysterious wall of flames consumes their survival packs. After many days, they find hope in the sight of green, living plants; water must be nearby. Suddenly, a quake swallows Judy. Frantic, the men move to higher ground in the hopes of spotting her and find a puzzling and frightening sight: ape faces carved into the mountainside. Their attention is drawn to a group of primitive-looking humans. They try to speak to them, but the primitives run away. Finally, Bill and Jeff collapse from exhaustion. The tribe takes the two men to their cave dwellings and nurse them back to strength. To their amazement, one of the tribe's women, Nova, is wearing United States astronaut I.D. dog tags around her neck. The tags belong to a Ronald Brent, an astronaut born in the year 2079, over a century after Bill, Jeff and Judy left Earth! With a little effort, Bill and Jeff manage to teach Nova their names. Suddenly, gas bombs are thrown into the caves and discharge. To their horror, Bill and Jeff see that the attackers are gorillas. Nova manages to hide Jeff, but Bill is captured along with most of the tribe and taken to Ape City in cages. Once there, Bill begins to fully realize that on this planet man is the animal and apes are the masters.