In Avatar, a man from an industrial society joins a low-tech tribal culture and becomes its leader. But the reverse happens too: Somebody from a disadvantaged group becomes a leader of the supposed masters. And here's how they do it.
There are many ways to go from desolate to despotic, but there are a few you'll see again and again in the annals of science fiction and fantasy.
Kings From The Pigsty
A classic scenario you find in fantasies and epics is the story of a humble person who rises to the top of his or her civilization. One of the best of the lot is assistant pig-keeper Taran in Lloyd Alexander's young adult series Chronicles of Prydain. Taran literally rises from the muck of the pigsty to become the king of all the land. Similar tales occur in Gene Wolfe's series The Book of the New Sun, in which a lowly, backwoods torturer rises up to become Autarch on an Earth that is slowly dying as the sun loses its power. In Piers Anthony's series Bio of a Space Tyrant a young man named Hope Hubris begins life as an oppressed refugee and eventually becomes Tyrant of Jupiter. And of course Riddick in the Chronicles of Riddick series goes from being a blind prisoner to ruler of the awesomely-named Necromongers.
These stories are not to be confused with the tales of lowly young people who become rulers from Star Wars or even the various tales of King Arthur. In those stories, we have a person of high birth who has been hidden or mistakenly brought low. Their journeys take them back to a pre-ordained "rightful place." That's very different from somebody like Taran or Hope Hubris, who have to earn and claw their way to the top.
I Came With Few Resources From Another World But I Still Rule Yours
Unlike Sully in Avatar, the heroes in stories like these come from other worlds without awesome technologies or giant squadrons of troops and scientists. John Carter, from Edgar Rice Burroughs' famous Mars series, is suddenly and inexplicably transported to Barsoom (AKA Mars). Despite language and cultural barriers, he still manages to become a great leader among the decaying advanced civilizations of the red planet. With just a sword and a few naked Martian ladies, he conquers all. Flash Gordon, like John Carter, is a product of the early twentieth century. He's somehow transported to the planet Mongo, and swashbuckles his way to the top of Mongo society using just his wits and his fists.
The main character in the first few novels of John Norman's bondage-happy Chronicles of Gor series is the nerdy but sword-mastery professor Tarl Cabot, who finds himself transported alone and without resources to the savage world of Gor. And yet he still manages to conquer! You can see the same scenario repeated in the original Stargate flick, where a small band of humans find themselves stranded on an alien world ruled by an advanced spacefaring race. Somehow the nerdy professor and his military pals manage to join up with the local population and bring down the ruling alien just by using their human pluckiness.
A Damaged Mind Shall Lead Them
Sometimes the lowly person is not an alien (or human among aliens), but instead a damaged human who somehow winds up becoming the leader of so-called normal humans. This is nowhere more evident than in Octavia Butler's superlative novel Mind of My Mind (part of her Wild Seed series), where Beverly Hills is taken over by a group of schizophrenics who live in the ghettos of Los Angeles. Turns out that these schizophrenics are actually powerful psychics, whose abilities have driven them mad - until a woman comes along whose psychic power is to organize their thoughts and help them channel their powers. Now they can control the minds of everybody who is rich or powerful, and bend them to their collective will.
On a less serious note, the fantasy of Forrest Gump (it is a fantasy, albeit one without dragons) is that a mentally disabled guy can become a powerful national leader and hero. And then there's the movie Idiocracy, where morons from the present day wake up in a future world where the media have dumbed everybody down so much that even the stupidest people of today are like geniuses.
Out Of The Past And Into The Driver's Seat
Speaking of idiots from the present day who conquer the future, let us contemplate the wonder that is the 1980s TV series Buck Rogers in the 25th Century. An astronaut/football player from the twentieth century is hurled 500 years into the future, and somehow manages to become the most important guy ever even though all his mores and abilities are completely outdated and superseded by everyone around him. Maybe it's just because he looks so awesome in those skintight white pants that people of the future favor.
In The High Crusade, by Poul Anderson, a band of English knights manage to hijack a spaceship and go on to conquer an intergalactic civilization despite their medieval ways. And in the movie version of Battlefield Earth (based on the book by L. Ron "Scientology" Hubbard), humans who have regressed to a pre-technological state manage to outwit and crush their alien slave-drivers, a group of hyper-technological aliens led by John Travolta.
Where Animals Rule
Though humans have ruled over all other animals on the planet ruthlessly for millennia, the other creatures' day will come. And that's exactly what happens in Battle for the Planet of the Apes, when the uplifted ape Caesar has led his enslaved simian brethren to victory over the humans. Eventually he decides to start enslaving humans the way they once did him.
A gentler version of this tale comes in Pixar's animated flick Ratatouille, where a lowly rat turns out to be the greatest chef in France - better than any human. At first he hides his rat nature by directing humans to cook for him, but eventually he comes out as a rat chef and is accepted by even the harshest of food critics.
Hulk has always been treated like an animal by humans, and that's why SHIELD decides to blast him into space so that they can be rid of the green menace. In the amazing Planet Hulk series by Greg Pak, however, Hulk finds himself on a planet called Sakaar where his brutal, animalistic nature makes him the greatest hero its people have ever known. He leads a group of oppressed aliens to victory over their cruel leaders, becomes King, and even takes a super-strong alien babe as his bride.
There are echoes of the "animals rise up" story in David Brin's two Uplift series, as well as in Poul Anderson's Slave Ship and Grant Morrison's comic We3. But in those stories, the animals don't wind up as great leaders of the humans who once enslaved them - they wind up as equals, possibly, or escape from slavery. But they don't really conquer.
Using Technology They Shall Rise
The main character in Iain M. Banks novel Matter, part of his Culture Series, is from a pre-industrial society that treats women like chattel. Luckily, she's adopted by a member of the Culture and rises to become one of the deadliest and most powerful members of Special Circumstances, an elite CIA-esque group whose job it is to affect the outcome of wars and other events on pre-technological worlds. She gets souped up with all kinds tech augmentation, and goes mega-ninja. Eventually she saves not just her own civilization, but all the other civilizations that live with it in a massive "nested world" created by aliens.
Another character who uses technology to become master of a civilization more advanced than his own is the videogame fiend in 1980s schlockfest The Last Starfighter. Turns out the videogame he's been pounding on is actually a test to become a member of an elite starfighting force that will beat back some evil aliens in another part of the galaxy. The master of the videogame is the master of the galaxy!
Neal Stephenson's terrific novel The Diamond Age follows the adventures of a street kid who accidentally finds the most powerful technology of her age: An artificially intelligent "educational primer" that teaches leadership as well as knowledge. Eventually she rises up out of the slums to lead a "mouse army" of other, similarly-educated girls, who may take over the world. And in Linda Nagata's novel The Bohr Maker, a prostitute who lives in a squat discovers a powerful form of nanotechnology, the Bohr Maker of the book's title, and uses it to completely transform the world. The developing world becomes the superdeveloped world overnight, while the so-called developed world finds itself in the ashcan of history.
Thanks to the HC and the humans of Twitter for ideas!