We're at the midpoint for the Planet of the Apes movie saga, with Escape from the Planet of the Apes! And once again, I'm "abusing" Wikpedia to get the plot summary from the Wiki page...
Super 8mm box art
The preceding film, Beneath the Planet of the Apes, ends with a nuclear weapon's destruction of all life on the apes' future Earth.
Escape from the Planet of the Apes begins by establishing that three apes (Cornelius, Zira, and Dr. Milo, played respectively by Roddy McDowall, Kim Hunter, and Sal Mineo) escaped the Earth's destruction. They have managed this by salvaging and repairing the astronaut Taylor's spaceship (which sank in the first movie) and piloting it through the shock wave of Earth's destruction sending the ship through a time warp back to the 20th century.
Belgian lobby card
The apes arrive on present-day Earth, splashing down on the Pacific coast. The navy hauls the ship to the beach, and the apes remove their helmets.
They are quickly transported to seclusion for examination and are later moved to a secluded area of the Los Angeles Zoo.
Milo is killed by a noncivilized gorilla who was agitated by an argument between himself, Zira, and Cornelius, leaving the two remaining apes under the observation of two scientists, Stephanie and Lewis. Both discover the apes' power of speech.
Meanwhile, a Presidential Commission has been formed to investigate the return of Taylor's spaceship and how the apes, which they already are aware are atypically intelligent, came to be aboard it. The apes then are brought before the Presidential Commission, where they reveal publicly their ability to speak, and are welcomed as guests.
The story of the plague that killed off all dogs and cats, as well as that of Ape slavery and subsequent uprising, is a possible retcon of both prior movies, wherein the apes do not know of their true past. Cornelius's claim that he had read history scrolls (kept secret from the masses) that detail the human downfall could be a way to rationalize the change, but fails to explain why he is as clueless as the rest of Ape society in the previous movies.
He may have had access to them, off screen, during the presumed months prior to Beneath, when Zaius made Cornelius his proxy, or while he and Zira were traveling in the spaceship. Another conflict is that, according to Cornelius, the day of Aldo speaking the word "no" is an annual observance in their society.
The apes become celebrities, being lavished with presents and media attention.
They are soon watched by a scientist, futuristic consultant Dr. Otto Hasslein (Eric Braeden), who discovers Zira is pregnant and fears for the future of the human race.
He is determined to force the issue, gives Zira a truth serum to get information out of her and convinces the Commission to have the apes taken for proper questioning. Both are questioned under numerous means; as a result, Hasslein learns for himself that Zira examined and operated on humans in the future.
Suspicion had already been aroused by Zira's letting slip, during public hearings, that she had dissected humans in the course of her work. More importantly, Hasslein learns how the human race will eventually meet its downfall and be dominated by simians, which will eventually lead to Earth's destruction.
After Cornelius accidentally kills an orderly (who had teased Zira) while imprisoned, Hasslein uses it as an illustration of the future danger the apes present and calls for the apes' execution. The U.S. President reluctantly orders that the unborn child's birth be terminated and that both be sterilized.
Running for their lives, Cornelius and Zira (assisted by Stephanie and Lewis) find shelter in a circus run by Senor Armando (Ricardo Montalban). There Zira gives birth to a son, whom she names Milo (later known as Caesar).
Hasslein, knowing Zira will imminently give birth, orders a search of all circuses and zoos. As a result, Armando must send the apes away. The drama climaxes aboard a derelict ship in an abandoned ship yard. Hasslein tracks the apes down, and finds Zira resting with her infant.
The arrogant and ruthless Hasslein shoots Zira in cold blood after she refuses to hand over her infant and then proceeds to fire several shots into the infant; he is immediately shot to death by Cornelius, who holds a pistol, and falls overboard. After Cornelius kills Hasslein, he is shot by a Marine Corps sniper and falls to the deck of the ship. Stephanie and Lewis watch in horror as Zira tosses her dead baby over the side of the ship before crawling to lie with her husband and dying by his side.
The survivors, however, are unaware of the real fate of the infant ape; Cornelius, Zira, and Armando having switched babies before their final escape. Armando now watches over the infant Milo, who will grow up to become Caesar, the protagonist in the third and fourth sequels, Conquest of the Planet of the Apes and Battle for the Planet of the Apes. The film ends by showing the baby ape Milo sitting in a cage, plaintively speaking the words "Mama? Mama?" with the voice of a human child.
Japanese movie program
This ending was purposely written so that the writers would have something with which to work, in case Fox Corporation wanted another sequel, although it was clearly stated in the DVD's bonus materials that this film was to be the end of the series and that Cornelius's speech should have made it full circle.
OK, back to me writing original stuff here. I'd forgotten that the whole disease that killed the cats and dogs thing, as well as the whole backstory of the Ape revolution, had been presented here! It does appear to be a continuity implant... but then, we don't really know for sure how much time elapsed between the first two movies, nor do we know how much time elapsed during the second movie!
The ape Milo must've been either a scientific genius, or like Cornelius, he ventured into things that were supposed to be forbidden to the Ape society... I suppose that it's possible that he came across printed materials that would've given him sufficient information to repair Taylor's ship, although I'd think it more likely that Brent's ship was discovered by Milo (perhaps with help from Cornelius and Zira), and pieces used from that to repair Taylor's ship. Given how damaged Taylor's ship was (plus the fact that it had sunk), it even seems unlikely it would've been repairable at all, but who knows?
Perhaps the lake that Taylor's ship crashed in periodically goes dry, and that's how that ship was recovered... or perhaps there was a delayed response from the inflation devices that most reentry vehicles should have, and so, it bobbed back to the surface again after Taylor & company abandoned it. Either way, you can either buy into the idea that the ship was repairable (however that was managed) and let the series continue, or you can say you don't believe it, and not enjoy the rest of the saga.
The overall tone of the movie is rather jarring... especially towards the end, when Cornelius and Zira are hiding in the shipyard, and their subsequent killing. By this time, your sympathies are directly aligned with the two apes, and you watch the movie hoping against hope that they'll escape Hasslein and certain death... but it's to no avail. You'd even think that the series would've ended as a trilogy, until you discover there's a whole bait-and-switch that we weren't privy to... and baby Caesar has, indeed, survived... which makes Caesar the Apes equivalent of Moses (something that would be thematically continued from here).
It's no wonder that, in Conquest, the human race becomes the bad guys, and the apes are the heroes!
A few additional thoughts: The disease that killed off the dogs and cats... it's possible that Cornelius, Zira, and Milo (as well as baby Caesar) were carriers of the disease... simians aren't affected by it, obviously, but they could've carried it, and therefore, it wasn't just Caesar coming back in time to spark the ape revolution, but the disease itself brought back (then again, the question arises, where did it originate?).
It's entirely possible that the disease was already being developed as a potential biological weapon at this time, and despite the warnings of Cornelius, it was continued in development, and being accidentally released.
Next week... Caesar grows up, and leads the Conquest of the Planet of the Apes!