Beneath the Planet of the Apes opens where the first film left off, with Cornelius' voice-over reminding everyone of man's blight upon the world. After a repeat of the last few minutes of the first film, the sequel kicks in with Taylor and Nova wandering through the Forbidden Zone. In the meantime, two astronauts have been sent on a mission from Earth to rescue Taylor's missing astronaut crew from the first film, and as bad luck would have it, crash-land on the planet of the apes. The year, according to the ship log, is 3955. One of the astronauts, known only as the skipper, dies, leaving John Brent (James Franciscus) to survive alone
After burying his comrade, Brent encounters Nova, who rides alone. When he mentions Taylor to her (he sees Taylor's dog tags around her neck), Nova remembers cinematically that they encountered great fire walls and earthquakes and that Taylor fell through what appeared to be solid rock. Brent and Nova make their way to Ape City, where they encounter Cornelius (this time played by David Watson) and Zira (Kim Hunter), who help the duo. They also overhear a plan by General Ursus (James Gregory) to expand Ape City by entering the Forbidden Zone and destroying a mysterious force that affects the senses of simians Brent and Nova are captured by solider gorillas, but with the help of Zira they manage to escape. The duo stumble upon an underground city, where Brent discovers that they really are on Earth — in New York City. This is the dwelling of mutant humans who worship an atomic bomb and have incredible power over other human minds, and soon Brent and Nova are captured. These mutants refuse to fight or kill even in defense. But they do not hesitate to use their powers to force others to do this for them. Taylor has been captured at this underground complex and is forced to fight Brent. Having survived 'ground zero' during the nuclear holocaust in humanity’s past, the telepathic beings are horribly scarred and mutated. Moreover, they have degenerated after so many years of suffering, having established a cult that worships the ultimate weapon, a cobalt-cased nuclear weapon Taylor later labels a 'doomsday bomb
