(Warning: As it is difficult to discuss adequately the Planet of the Apes series without giving away some important plot information and potential surprises (especially in the first film), the following article contain numerous spoilers. If you have not yet seen the films, proceed at your own risk.)

From 1968 to 1973, five Planet of the Apes films were made and released, which, by any stretch of the imagination, is a prolific outpouring of cinematic production. What is surprising, though, is that the sequels consistently took the story in new and interesting directions, rather than telling the same narrative over and over again, as most sequels do. Granted, the four sequels are all lesser movies that the original Planet of the Apes (1968), and their continuously decreasing production values become more and more evident with each new entry into the series (the fifth film was budgeted at less than one-sixth of the original). However, when seen together, these five films represent a landmark in science fiction: the first sci-fi movie series.

The first in the series, Planet of the Apes (1968), begins aboard an American spaceship that is returning home after six months of exploration in deep space. The ship crash-lands on what appears to be an alien planet some 2,000 years in the future (the dialogue laboriously refers—over and over again—to Einstein's theories of time warping in light-speed travel). Three astronauts survive, led by George Taylor (Charlton Heston), a grumpy misanthrope who bemoans the violent tendencies of the human race. At one point, he says he joined the space exploration team in the hopes of finding a better species.

Instead, he finds himself marooned on what he thinks is a distant planet in which evolution has gone backwards. Apes are the highest form of life, with a complex, if somewhat rustic, civilization. They don't have any complex technologies or machinery, but they discuss science, religion , and logic while also adhering to a system of segregation in which different simians hold higher social positions (orangutans are the politicians, chimpanzees are the intellectuals, and gorillas are the labor and military). On the other hand, humans are unevolved, animalistic wanderers who cannot talk. In other words, what we consider human attributes are what characterize the apes, and vice versa for the humans.

Like all the best science fiction, Planet of the Apes is about much more than its surface narrative. Science fiction has always been a great means for examining the human condition because it lends itself so easily to allegory. The necessity of introducing fantastic pseudoscience, radically advanced technology, and alien creatures provides an easy framework for disguising stories that are really about ourselves. Thus, as Invasion of the Body Snatches (1956) is really about the '50s Communist scare and War of the Worlds (1953) is really about the fear of foreign invasion, Planet of the Apes is about one of the great debates of human history: faith versus science.

We are introduced to several major ape characters, including the sympathetic behavorial scientist Dr. Zira (Kim Hunter) and her fiancée, an archaeologist named Cornelius (Roddy McDowall). They have a theory that apes evolved from humans, and they think they have found the missing link in Taylor. Dr. Zaius (Maurice Evans), who serves in the conflicting dual role of both Minister of Science and Protector of the Faith, will hear nothing of it, and he accuses Dr. Zira and Cornelius of "scientific heresy." Apparently, the ape civilization is quite dogmatic, adhering strictly to a set of sacred scrolls that were handed down from the great, mythic "Lawgiver" 1,200 years ago. According to the scrolls, apes are superior, humans are inferior, and that's the way it always has been and always will be.

Sound familiar? The allegorical aspects of Planet of the Apes are almost too obvious, yet they are highly effective. Placing human stories in nonhuman terms allows for a new perspective on what seems a tired argument. In watching Dr. Zaius blithely condemn others who are obviously right, all in the name of protecting the faith, brings back painful memories of similar behavior in human history, from Socrates being forced by the citizens of Athens to drink poison because he was accused of denying the gods, to Galileo, under threat of death from the Roman Catholic Church, having to recant his correct observation that the Earth revolves around the sun.

Planet of the Apes was based on the novel Monkey Planet by Pierre Boulle (who also wrote Bridge on the River Kwai), and it was adapted by Michael Wilson and Rod Serling, two writers who fit the material perfectly. Wilson, who was a prolific screenwriter in the 1950s (he won the 1951 Oscar for A Place in the Sun) before being blacklisted during the McCarthy era, knew exactly how dogmatic adherence to any set of principles can lead down the path of destruction. In particular, the scene in which Dr. Zira and Cornelius attempt to defend their position in a "hearing" has HUAC written all over it, although it can also be seen as a variation on the Scopes Monkey Trial of 1925, in which a Tennessee biology teacher was tried in court for teaching Darwin's Theory of Evolution.

Rod Serling was best known as the creator of The Twilight Zone, a science-fiction TV series that was almost always allegorical in nature and featured shocking, ironic endings, both of which characterize Planet of the Apes. One of Serling's major contributions to the screenplay was the infamous ending in which Taylor finds himself facing the crumbling Statue of Liberty, proving that not only has he been on Earth the whole time, but that his worst fears about the human capacity to destroy itself have been realized.

Of course, Planet of the Apes is an obvious product of the late '60s (one teenage ape character is a rather pathetic concession to the youth movement, complete with lines about not trusting anyone over 30), and it has sustained its share of jokes over the past 32 years. (On a side note, The Simpsons TV series has featured some of the more inspired parodies of the movie, especially one episode that featured a big Broadway musical titled Stop the Planet of the Apes! I Want to Get Off, which featured a song about Dr. Zaius set to the tune of Falco's kitschy '80s techno-hit "Rock Me Amadeus.") John Chambers' ape make-up, which was so heralded in 1968 that it won a special Academy Award, is still quite impressive, although its rubbery limitations have become more obvious over the years.

Still, the film was a huge success at the time, and along with Stanley Kubrick's 2001: A Space Odyssey, which was released the same year, it was largely responsible for the mainstreaming of science fiction as more than B-movie material, thus paving the way for Star Wars (1977) and other sci-fi hits of the last 30 years.

Because Planet of the Apes was such a huge financial success, it was little surprise that several sequels followed. Two years later, Beneath the Planet of the Apes (1970) was released, and it is a markedly different film. While Planet of the Apes ends on a surprise note that could certainly be construed as downbeat, Beneath tops that ending shocker with one that can never really be topped: the destruction of the entire planet. It says a great deal about the movie business 30 years ago that a major studio picture with a great deal riding on it could be so dark and end with all the major character being killed right before planet Earth is utterly decimated.

The main character in Beneath the Planet of the Apes is an American astronaut named Brent (James Franciscus, who seems to have been hired for his physical resemblance to Charlton Heston). Brent follows Taylor's path to the future where apes rule Earth, and he stumbles upon a subterranean city that is populated by mutant humans with mind-control abilities who worship a doomsday atomic device designed to destroy the entire planet in case of nuclear war. The ape civilization, fearing the existence of another group in the Forbidden Zone, mounts an invasion that ends in complete destruction.

Beneath is different than its predecessor in that it is much more militaristic in tone (which foreshadows the last two films in the series). There are explicit allusions to the war in Vietnam, which was at full scale when the film was released. In one scene, a group of peaceful chimpanzees stage a protest against the gorilla military, complete with peace banners and nonviolent protest in the form of blocking the street. The director, Ted Post, even goes so far as to switch to a hand-held camera to give the scene a violent news footage feel as the gorillas forcibly remove the protestors.

The film also focuses a great deal more on the horrors of nuclear holocaust (an enduring fascination of science fiction since World War II). While it is only suggested at the end of Planet of the Apes, Beneath solidifies the fact that humankind destroyed itself with nuclear war, thus allowing the apes to evolve and take over the planet. The film is replete with images of destroyed 20th-century civilization. The famous half-buried Statue of Liberty is joined by the ruins of the New York Public Library, the New York Stock Exchange, and Radio City Music Hall, most of which has been buried underground.

The majority of the story takes place in the dank ruins of the subway system, giving an overall sensation of claustrophobia. While the production values aren't nearly as good as the original (the sets are very obviously sets, and the matte paintings are sometimes embarrassingly one-dimensional), Beneath the Planet of the Apes is still an effective sequel that takes an increasingly dark look at the unavoidable nature of war.

The next film in the series, Escape From the Planet of Apes (1971), essentially reverses the scenario of the first film by taking two familiar ape characters and stranding them on Earth in the present day (well, 1973, anyway). By a narrative conceit, Zira, Cornelius, and another ape, Dr. Milo (Sal Mineo), had salvaged Taylor's crashed spaceship from the first film and managed to fly it off the planet right before it was destroyed at the end of Beneath the Planet of the Apes.

For whatever reason, they travel back in time and land in the ocean off the coast of California. They are immediately taken to the Los Angeles zoo, where they are observed by Dr. Lewis Dixon (Bradford Dillman) and Dr. Stephanie Branton (Natalie Trundy) in one of the film's funniest sequences. It doesn't take long for the humans to realize that these apes are intelligent and cultured, so naturally they become the toast of the Los Angeles scene. Cornelius gets outfitted in a three-piece suit, Zira gets involved in women's liberation, and the press follows them everywhere (the social satire is so thick one can literally cut it with the proverbial knife).

Troubles arrives, though, when Zira admits to government officials their origins in the future, where simians dominate humans and the Earth is eventually destroyed through warfare. An ambitious presidential science advisor, Dr. Otto Hasslein (Eric Braeden), becomes determined that, if Zira and Cornelius are killed, then the future of ape domination will be avoided. When Zira becomes pregnant, he becomes even more intent on destroying them, as he realizes her progeny will be the seed of the later dominant species.

Although some claim it is one of the best, Escape From the Planet of Apes is my least favorite of the series. It suffers mainly because it loses the scope and scale of the other films; it feels like a quickie insert that is needed to progress the larger narrative, but doesn't have enough meat on it to deserve its own film. It gives a great deal of information in dialogue that is then fleshed out in detail in the following two films, which unfortunately makes the last two entries in the series seem somewhat unnecessary since we know what will happen.

Having two apes on present-day Earth is, after the first few minutes, a rather dull scenario, and the film attempts to make up for it with more humor and whimsy. Nevertheless, the story turns more serious in the last 20 minutes and still manages to end on a down note, with multiple deaths of major characters. However, the clever screenwriter, Paul Behn (who also wrote or contributed to the screenplays for all the Apes films save the first one), leaves the door open at the end by making it clear that, even if Zira and Cornelius didn't survive, their child did.

Conquest of the Planet of the Apes (1972) is often referred to as the least of the series, but I think it is one of the more effective films. Moving away from the somewhat goofy tone of Escape From the Planet of the Apes, Conquest shifts back into darker territory as it depicts the events that led up to the downfall of humankind and the rise of the apes.

The film takes place some two decades after the events in Escape (a title card lists the year as 1991, which was, at the time the film was made, not too far into the future). Apes are now used as slave labor in a vaguely futuristic human society that seems to have moved in a totalitarian direction. Apes are beaten and denied rights, made to do menial tasks and hard labor, and sold at auctions to the highest bidder (the parallels with the slave treatment of African-Americans are strong, making this the film the most racially allegorical).

However, Caesar, the child of Zira and Cornelius, raised by the kindly circus master Armando (Ricardo Montalban), lives. And, because he has advanced intelligence and capacity for speech, he is able to gather the apes together and lead an uprising. Played by Roddy McDowall in much different fashion than he played Cornelius, Caesar is a complex central character. While at first he seems sympathetic, forced to hide his intelligence to avoid being destroyed by a paranoid government, he is, in the end, a frightening vision of power, leading a bloodthirsty army that will follow his every command. His final speech in the waning minutes of the film, in which he declares his intent to destroy humankind and turn Earth into the Planet of the Apes, is a chilling indictment of how infecting power can be. Even a lame overdubbing in post-production that includes baloney about "domination with compassion" cannot quite take the razor edge off McDowall's scathing delivery.

Conquest of the Planet of Apes is notably more violent that its predecessors, featuring scenes of torture by electrocution and an extended riot sequence in the heart of a major city in which apes overpower armed policemen. Like the earlier Apes films, these sequence are disconcerting in how they are framed to match counterparts in real life, most notably social revolutions in unstable countries around the world, as well as race riots in the United States (in fact, director J. Lee Thompson modeled these scenes after newsreel footage of the 1965 race riots in Watts). The message is that passing power from one to another does not change the essential nature of power. Rather, power contaminates the person (or ape) who wields it.

The series was finally brought to a close with Battle for the Planet of Apes (1973), which takes place roughly a decade after the events in Conquest. In this narrative, nuclear war has ravaged most of the Earth, and apes have begun to assert their superiority. Although they still live together with some humans, the humans are treated mostly as slave labor and inferior creatures.

Roddy McDowall returns as Caesar, the leader of the apes who has consolidated power and is trying to make a better world (his character has softened a great deal in the off-screen passage of time). The first commandment of his society is that "ape shall not kill ape," but their mostly peaceful existence is under constant threat of collapse as the war-hungry gorillas, under the command of General Aldo (Claude Akins), push for violence as a means of asserting authority. War erupts when a group of mutated humans from the radioactive wasteland of the fallen cities invade the city of the apes. The scene depicting the humans riding out of the ruins in burned-out Jeeps and tanks is a striking one, and it must have been an inspiration for George Miller's Mad Max films (1979-1985).

Although it sounds as dark as Conquest of the Planet of the Apes, Battle is actually the only film in the series to end on a positive note. The final scene suggests that humans and apes have learned how to live together in a peaceful coexistence, thus negating the future discovered by Taylor in the first film. Legendary director John Huston makes a brief appearance at the beginning and end of the film as the Lawgiver, but instead of handing down proclamations about the superiority of the simian species, he tells the tale of noble Caesar and how he created a beneficial society out the wreckage of nuclear war.

Thus, the Planet of the Apes series is essentially brought full circle. Its political and social commentary, while often scathing, comes to a close with a hopeful outlook for the future. Over a span of six years and five films, the Apes series spun a magnificently bold story of evolution. While the fabric of its narrative was certainly wearing thin by the end, this series will always remain an important landmark and a wonderful example of how entertainment value and political commentary can be woven together with great success.