Flipside to the First 'Apes' Movie
In a lot of ways, Escape from the Planet of the Apes is the flipside to the original Planet of the Apes film. In this installment, Zira and Cornelius and Caesar crash land in the 20th century where they are initially treated like animals, until a sympathetic animal psychologist learns of their intelligence, just as they once did for Taylor in the first film. Also in a reversal of the first film, their presence threatens the military and religious status quo, and they ultimately have to flee for their lives.
It's this social commentary that makes 'Escape' more than just a throwaway entry in the series. While it's not exactly new territory, it's at least entertaining, and the setup for the original 'rise' of the planet of the apes.