Following the events of previous 'Planet of the Apes' movie, time-displaced astronaut George Taylor (Charlton Heston) and the mute human Nova (Linda Harrison) are riding on horseback through the desert of the Forbidden Zone. Suddenly and without warning, walls of fire spring up from the ground and deep chasms open in the earth. Confused by the strange phenomenon, Taylor investigates a cliff wall and disappears before Nova's eyes.Elsewhere in the Forbidden Zone, a second human spaceship has crash-landed after being sent in search of Taylor and his crew. Like Taylor's ship, it has traveled forward through time into Earth's distant future. Two astronauts emerge whom are John Brent (James Franciscus) and the gravely wounded captain of ship, known only as Skipper, who soon dies from his wounds. Brent reads the Earth year on his crashed ship as 3955 A.D., and he believes he has traveled to another planet, unaware that this is Earth of the far-future. He encounters Nova still riding on Taylor's horse and notices she is wearing Taylor's dog tags, but he cannot get her to speak.Hoping Taylor is still alive, Brent rides with Nova to Ape City, where he is shocked to discover a civilization of bipedal, talking apes. He observes the militarist gorilla General Ursus (James Gregory) leading a rally calling for the apes to invade the Forbidden Zone, conquer it, exterminate all humans, and use it as a potential food source, against the objections of the orangutan ruling leader Dr. Zaius (Maurice Evans) as well as the chimpanzee minority of Ape City. Brent is spotted and wounded by a gorilla soldier. He is taken by Nova to the home of Cornelius (David Watson) and Zira (Kim Hunter), who treat his wound and tell him of their previous encounter with Taylor. The two humans hide when Dr. Zaius arrives and announces that he will accompany Ursus on the invasion of the Forbidden Zone.Attempting to flee the city, Brent and Nova are captured by gorillas. Ursus orders they be used for target practice, but Zira helps them escape. Brent and Nova flee into the Forbidden Zone where they hide in a cave which Brent discovers to be the ruins of the Queensboro Plaza station of the New York City Subway, sparking the realization that he has traveled to Earth's post-apocalyptic future. Following a humming sound deeper into the underground tunnels, Brent begins to hear voices telling him to kill Nova. Entering the remains of St. Patrick's Cathedral, he finds a population of telepathic humans who worship an intact nuclear bomb.Meanwhile back in Ape City, Dr. Zaius prepares to accompany General Ursus for the invasion by several armed gorilla battalions into the Forbidden Zone to exterminate any humans there. They are temporarily halted by a peace protest by the chimpanzee minority whom the gorillas proceed to detain. As Ursus and Zaius lead the huge ape army away from Ape City into the desert, Cornelius and Zira plot their own excursion into the Forbidden Zone desert to look for Taylor's spaceship by the Dead Lake deep in the zone where he told them in the previous movie where he crashed it.Back in the underground city, Brent and Nova are captured by the mutant humans and are telepathically interrogated. Brent reveals the apes are marching on the Forbidden Zone and will find their underground city. Above ground, the telepaths attempt to repel the ape army when it arrives by using their mental abilities to project illusions of fire and other horrors, as they had done to Taylor and Nova. Dr. Zaius sees through the illusions, however, and leads the ape army through to the ruined city. With the apes closing in, the telepaths plan to detonate their "Divine Bomb" as a last resort. They hold a religious ceremony, at the height of which they remove their masks to reveal that their appearance has been grotesquely mutated by centuries of exposure to nuclear fallout.Brent is separated from Nova and taken to a cell, where he finds Taylor imprisoned. The mutant Ongaro (Don Pedro Colley) uses his telepathic powers to force Brent and Taylor to fight each other to the death. Nova escapes her guard and runs to the cell, screaming her first word: "Taylor!" This breaks Ongaro's concentration, freeing Brent and Taylor from his control, and they overpower and kill him. Brent describes the bomb the mutants worship and Taylor recognizes it as a "doomsday bomb", capable of destroying all life on the surface of the planet.The heavily armed apes invade the subterranean city, killing Nova and making their way to the cathedral, shooting, stabbing, and clubbing all the mutant humans they see. They are confronted by Méndez (Paul Richards), the mutant leader who raises the bomb into activation position before being gunned down. Brent and Taylor attempt to stop Ursus from accidentally setting off the weapon, but Taylor is shot. Brent manages to take a rifle off a dead gorilla where he personally kills Ursus and several of his gorilla soldiers before being overwhelmed and shot dead by the other gorillas. The mortally wounded Taylor pleads with Dr. Zaius for help, but Zaius refuses, saying that man is only capable of destruction and that all humans must be exterminated. In his last moment, Taylor defiantly brings his hand down on the activation switch, triggering the bomb, which detonates... thus wiping out all life on the planet.
Output: Question: Where does Brent find the population of telepathic humans?, Answer: the remains of St. Patrick's Cathedral