I've never liked this sequel as much as the first sequel, I think because it's such a downer. Taylor, who escaped with the mute girl Nova at the end of the first movie, travels further into the Forbidden Zone on horseback and seemingly disappears into thin air. Nova remembers that if anything happened to Taylor she was to go back to Zira and Cornelius for help. Along the way she runs into John Brent (James Franciscus), another astronaut who has crashed. They go to Ape City and he's shocked at what he sees. He meets Zira and Cornelius, they try to help them but they're captured by the militant gorillas, who want to use them as target practice. Zira unlocks the cage they are in and they manage to escape and head back to the Forbidden Zone. There, Brent finds an underground cave that turns out to be the ancient NYC subway system and finds himself in the remnants of Queen's Plaza. Walking further, there's the New York Public Library and a few other landmarks. He can't believe it, of course. Even weirder, there are intelligent humans there that worship the Alpha/Omega nuclear bomb as if it were a god. During their church services they reveal their true faces--and it turns out they are all hideously deformed from the thousands of years of radiation. However, it has also given them psychic abilities and they force Brent and the captured Taylor to fight to the death. They nearly do but manage to kill the mutant and escape. However, this is just as the ape army is invading, so it's bad timing. The mutants begin to activate the "doomsday bomb" and Brent stops them, only to be shot in the head by one of the gorilla soldiers. Then, Taylor is shot--and falls against the controls, his dead hand setting off the detonation.
I never realized it before seeing the BEHIND THE PLANET OF THE APES documentary that a different actor portrays Cornelius here (David Watson), as Roddy McDowell was off directing his own film at the time. Also, it still amazes me that this was rated "G" when it was released forty-one years ago.