With 1972's Conquest of the Planet of the Apes, the saga completed its circle.

Following 1971's Escape From the Planet of the Apes, the third of five movies in the popular series, a reboot of sorts was required. The protagonists of that film, friendly chimps Cornelius and Zira, were the only holdovers from the original 1968 outing, but their deaths at the hands of a government fearful of a future with talking apes left a hole in the continuing narrative.

Enter baby Milo, Cornelius and Zira's child who was rescued by a kind circus leader at the end of Escape. Milo grows up to be Caesar, an increasingly rebellious talking ape in the year 1991. It's not long before he kicks into motion the main narrative of Conquest of the Planet of the Apes by shouting "Lousy human bastards!" as he watches a helpless gorilla beaten by sadistic humans.

The seeds of revolution now planted, Caesar (played by Roddy McDowall, who starred as Cornelius in two of the preceding three films and portrayed another chimpanzee, Galen, in a 1974 Planet of the Apes television series) begins to prepare his fellow apes for an uprising. Caesar is sold at auction to Breck, a governor who's suspicious of apes, but the chimp warms to the official's sympathetic assistant MacDonald, a Black man who shares the chimp's feelings about his slimy and borderline racist boss.

Caesar is eventually saved from death by MacDonald and begins his revolution in earnest, first freeing captive apes and then arming them for a takeover. The subsequent riot culminates in deaths, citywide fires and Caesar marching Breck to a public square for execution.

McDowall's character then gives an impassioned speech that echoes post-civil rights movement frustrations: "Where there is fire, there is smoke. And in that smoke, from this day forward, my people will crouch and conspire and plot and plan for the inevitable day of man's downfall. ... We shall build our own cities, in which there will be no place for humans except to serve our ends. And we shall found our own armies, our own religion, our own dynasty. And that day is upon you now!"

Watch the Trailer for 'Conquest of the Planet of the Apes'

It's no accident Conquest of the Planet of the Apes arrived at this point during a similar upheaval in American culture. The impact of the race riots that had torn through U.S. cities at the end of the '60s was still being felt, and prisons were becoming overcrowded with mostly Black men. The parallels were easy to connect.

Director J. Lee Thompson and screenwriter Paul Dehn may have been journeymen filmmakers whose gigs included everything from quickie TV movies to James Bond films, as well as other Planet of the Apes entries, but they delivered an often subversive, if occasionally clumsy, fourth outing in a science-fiction series that was increasingly being targeted to kids.

That's one of the reasons the film's original ending, where the apes kill Breck, was softened to show them having second thoughts following an additional speech by Caesar, which McDowall later recorded to appease producers who were angling for a more child-friendly rating. (A pre-title sequence, in which an escaped ape is shot and is revealed to have been abused, was cut for the same reason.)

Just as important, though, Conquest of the Planet of the Apes brought the entire story full circle. While the film doesn't fully explain how the events in the first movie transpired, it does offer a sense of how they were set in motion by the ape revolution seen in Conquest. Of course, more suspension of belief than usual is required to wrap your head around the concept that two talking apes from the future landed in 1973 and eventually brought simians to power before a nuclear bomb annihilates everyone ... except for two talking apes from the future who landed in 1973.

Little of this made a difference to fans swept up by Apemania. Following its release on June 30, 1972, the movie, like its predecessors, debuted at No. 1, opening the production door for the fifth and final film in the original series, 1973's dreadful Battle for the Planet of the Apes. Comic books, action figures, trading cards and TV shows followed. Nearly three decades later, Conquest's main plot point served as the basis for the franchise-rebooting Rise of the Planet of the Apes, its social-justice themes just as relevant in 2011 as they were in 1972.

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