Sometime after the events of the first Planet of the Apes, the climax of which is repeated frame for frame at the beginning of this sequel, another set of astronauts arrives on the far-future Earth that is the titular planet.
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I love this movie way more than I ought to. It's barely an apes film at all - it's practically the first film, with psychic mutants as the enemies instead of apes. It's more like one of those off-the-wall 1970s apocalypse movies like Glen and Randa or A Boy and his Dog than a sequel to Planet.
I'm not going to say this movie is amazing or anything... but I really kind of dug how fucked up it was. Any movie that has an underground cult that worships the nuclear bomb in it is good in my books.