Originally Charlton Heston had hoped that by ending Beneath the Planet of the Apes in the way he did, it would bring an end to further Apes sequels. His plan didn't quite have the effect he wanted. Instead Fox created a second sequel - Escape from the Planet of the Apes - to begin a second major story arc and open up the possibility of more films in the series being produced.
In Escape, Cornelius and Zira the chimpanzee scientists who protect Taylor and Brent in Planet and Beneath, escape the soon to be destroyed Earth on Taylor's spacecraft, which they have repaired with the help of another chimp scientist. They hit a space time disturbance and are shunted back in time to the present day - just a couple of years after Taylor and his crew left Earth. Unlike when Kirk and Spock came to town, there's no whale stealing and the two apes become celebrities. Ultimately the early ape-human friendship gives way to mistrust after it is revealed what future lies in wait for the human race.
Escape is the most light-hearted film in the series. The ending is rather gruesome but it's the least philosophical and the least political of all the Apes. The themes, as in many films of the era, reflected America's presence in Vietnam. America feeling threatened by an invisible, foreign enemy - the Apes when they evolve - and then subsequently trying to secure humanity's future by attempting to quell any chance of a perceived uprising further down the line. It throws up a couple interesting quandaries - Zira's dissection of humans for research being the most thought provoking. Really though, it's just a big what if...?
Escape requires a great amount of suspension of disbelief from the viewer. Especially the way the film sets up the apes arrival on Earth. After being shocked at the miraculous sight of a paper airplane gliding through the air - somehow in the short space of time between the end of the first and second film, they have mastered engineering to the point where they are able to fix Taylor's destroyed shuttle and then quickly gain the ability of spaceflight. It's a hard sell but Escape does kind of work, it's an enjoyable film though in reality just a gateway for further films to be produced down the line. The best part may be the return of the twist ending, which Beneath dispensed of.
The ending forms the films biggest talking point. In Escape it is both brilliant and cynical. Some have argued it effectively destroys the original film's timeline and creates an alternative one, which is probably true. Zira and Cornelius are killed but they manage to secure the life of their newly born offspring - who in the films final shots, demonstrates his burgeoning mind. The threat to the human race has not been eradicated. It's a clever ploy but one can't help but think it was thought up by studio executives, looking to make more money off a cheap and relatively profitable franchise.
By saving the life of baby Milo, the process by which Apes begin their ascent is sped up. The stage set for Conquest of the Planet of the Apes...