"Beneath" is a film that epitomizes the best and worst that a sequel to a masterpiece, as "Planet Of The Apes" was, could be. For me, it is the most watchable of the sequels only because it's the only one that stays in the familiar territory established by the first film. But oh that ending....
I really can not understand why the heck Richard Zanuck allowed Charlton Heston to talk him into going with that downer ending that has already been referred to by others. The only reason why Heston pushed it wasn't because it made for good cinema but because he wanted to singlehandedly see to it that he never have to do another Apes sequel again. It's a pity that Heston never realized that the best sequel story one could have explored was what happened to Taylor and Nova afterwards, and could have made for some equally compelling drama as the first film did. To me, the main appeal of the first film wasn't the Ape society, it was the character of Taylor and his fascinating odyssey from misanthrope to defender of the species, only to see his earlier feelings borne out in the shock ending. The question that should have been addressed next in a sequel was, what would Taylor do now that he realized he was on Earth? What other things were there to discover on the planet? (Heck, you could have easily made a sequel movie without the apes returning, as far as I'm concerned!) Alas, because of the plot device imposed by this film (and Heston's inability to realize what a good thing he might have had going for himself), the rest of the series had to veer off into what was for me, a more uninteresting story line focusing on the Apes and their inevitable rise to power.
Plot holes and depressing ending notwithstanding, "Beneath" is still worth watching to see the traces of what could have been a great film that are there. The mutant storyline and their dwellings underneath the remains of New York still has an irresistible viewing quality to it. I do not recommend watching it in tandem with its magnificent predecessor though, because that's only going to make you more angry about the ending of this film.
