- "The Forbidden Zone was once a paradise. Your breed made a desert of it, ages ago."
- ―Zaius to George Taylor
The Forbidden Zone was the radiation-disfigured land throughout the Apes series. A Forbidden Zone could be a mountainous desert or swamp, city or sea. It just had to be deadly as well. It would seem that each Forbidden Zone has to maintain a certain quota of Mutants also.
The Forbidden Zone is first seen when George Taylor's spaceship crashes into a lake. After escaping from their sinking ship, Taylor, John Landon, and Dodge walked through great stretches of mountains and desert, at last finding an oasis guarded by what appear to be humanoid scarecrows.
Later on, Cornelius I, Zira, and their nephew Lucius free Taylor and Nova where they flee into the Forbidden Zone to prove that man was once superior to the ape. At an archaeological site by the sea, Dr. Zaius and the Gorilla Soldiers catch up to them since "only an apostate would flee into the Forbidden Zone." Taylor manages to capture Zaius and within the cave, they discover a human doll that talks, thus proving that humans once held superior technology and that apes somehow descended from them.
Dr. Zaius allows Taylor and Nova to go further into the Forbidden Zone stating that Taylor may not like what he finds out there. While charging Zira and Cornelius for heresy and going back on his word, Dr. Zaius arranges for the cave to be blown up.
Taylor and Nova are shown wandering further through the Forbidden Zone when Taylor suddenly disappears and is captured by Mutants.
Meanwhile, Dr. Zaius and General Ursus plan to lead their army into the Forbidden Zone to destroy the force that affects the simians' senses.
- Like many aspects of the Planet of the Apes mythos, the concept of the 'Forbidden Zone' evolved gradually through a number of years and a number of writers. Pierre Boulle's original crew of astronauts glided over modern towns and roadways before coming to rest on a plateau clearing within a jungle. The dense jungle provided the only cover for the spaceship, and the downfall of humanity was due to to their own mental and physcal lethargy rather than to nuclear holocaust.
- When Rod Serling adapted the novel to a screenplay, he first introduced the notion of a radioactive wasteland. This, in turn, would provide a perfect hiding place for the astronauts' advanced technology. In Serling's draft scripts, the ship automatically touches-down in a valley within a mountain range - hidden even from the watchful helicopters Serling's apes used for transport. The crew travel across a body of water to reach the more promising verdent lands beyond. Later, an ape reporter questions Thomas about the whereabouts of his craft: "We've been told that your ship landed in the Island area which has been quarantined to our race for many centuries." At an ape university, Thomas is told some more about this 'quarantined area' when he asks why the apes inhabit only a tiny part of the land-mass. Dr Ernestine of the Biology Department replies: "Did anyone explain the contamination to you, Mr. Thomas?... Expeditions have been sent out, Mr. Thomas - west on our own continent and to several of the continents beyond. There has been evidence of some form of radio-activity. But of a massive nature... It's mentioned in many of our historical documents. Places where plant life and human life existed in abundance, totally wiped out. So we've been rather cautious in our expeditions beyond." The reason for all this is spelled out to Thomas at the archaeological dig, where a nuclear fallout shelter is unearthed, holding the skeletal remains of advanced human beings. The jungle area the astronauts had crossed to, and where they were captured, was named by Dr. Zira when she asked the hunt-leader, Mr. Digby, about his "expedition into the inner-land".
- Writing the script ultimately used in the movie, Michael Wilson took the framework of Serling's treatment and altered the dialogue and the advanced nature of the apes, but maintained the basic story. Thus Serling's idea about a barren man-made wilderness survived, but with a new and ominous-sounding name - The Forbidden Zone.