Beneath the Planet of the Apes is terrific sci-fi flick, less epic than its predecessor but filled with some of the same great imagery with the added zest of now the viewer knows the planet is a demolished Earth of the future. BtPotA is the only one of the five Apes films which was adapted to comics at the time of its release, getting the treatment by Gold Key. Later of course Marvel would take the license and do some nice things, but the cogs moved slowly it seems at first for cross promotion for this series. It you'd like to read this adaptation featuring artwork by the Alberto Giolitti studio then check this out.
I pretty much assume most folks know the plot of this one -- a second astronaut lands on the planet of the Apes in search of the first and finds not only a hostile ape society but an even more hostile society of mutated humans who worship the atomic bomb which demolished civilization as we know it and irradiated the landscape.
James Franciscus is a likeable leadng man, who served well in the mostly B-level pictures and television shows he appeared in. Lacking the gravitas of Charlton Heston, Franciscus as the astronaut Brent arrives and searches for the lost team which preceded him and eventually finding the demoralized Taylor ("Damned-dirty-Ape" Heston his own self) who had earlier disappeared in the "Forbidden Zone". Brent is helped by mute Nova (Linda Harrision) and Zira (Kim Hunter). (Zira's ape husband Cornelius is played is not played by star Ape alumnus Roddy McDowall who was committed to other projects, but by David Watson.) There is much action, much of it intended to echo what had already appeared in the dynamic first movie before Brent stumbles across the ruined subway systems of NYC and the mutants who can cast illusions with their minds. The signature scene is those mutants, well turned out for an apocalypse worshiping at the base of an unexploded nuclear rocket. The finale is a brutal one with little hope for the eventual salvation of man or ape.
So the continue to milk the cash cow the Apes movies had become we know of course that with the third one (Escape from the Planet of the Apes) that Zira and Cornelius escape into the past and we follow the inevitable history which will take us to the stark moments of decision we see here. The Ape films are surprisingly popular given the dour nature of their themes.