[Note: If you don't want Oblivion spoiled, you probably shouldn't read this. However: knowing the movie's plot beforehand probably won't diminish the experience of seeing the movie--it might enhance it, really, because you won't have to wonder what the hell is going on the whole way through.]
It's something of a spoiler to call Oblivion post-colonial sci-fi. We don't learn until the last 30 minutes of the film that aliens (or maybe an AI?) have colonized and almost completely depopulated Earth. The last dregs of humanity cling tenuously to life below the planet's surface. The movie's narrative twists and turns and strains credulity, but the setting can be easily understood: a powerful space-faring society has taken control of Earth for its resources, driven a small resistance force of humans underground, and turned the surface of the planet to desert.
Almost nothing in Oblivion is new. The post-colonial setting I described has been explored in science fiction over and over again. The plot points and the images the film employs have often been yanked whole from other, better movies*. Jack Harper, played by Tom Cruise, is a lot like Wall-E--though less lifelike. Yet, hidden in the pastiche, there's something unsettling about Oblivion. Here it is: Oblivion is a movie about how Jack Harper becomes a suicide bomber.
"Terrorist" is a difficult word, but its the best one I have to describe Jack Harper, who employs the tactics of modern terrorism. At the end of the film, Harper pilots a vessel into a strategic location and detonates a nuclear bomb, martyring himself in the hope of shocking the aliens into leaving--in other words, martyring himself to achieve a political objective. The specifics of the scenario hit pretty close to home, too: a great military power wages a drone campaign against a technologically inferior enemy, which spawns a resistance. The big difference between Oblivion and America in 2013 is, in Oblivion, the good guys are the terrorists, and the bad guys have the drones. Is Oblivion a criticism of U.S. foreign policy? Of drone strikes? Is it a pre-emptive criticism of the police state the world might become?
I'm not so sure the film wants to be any of those things. This is a summer blockbuster, after all. But at its heart, Oblivion is a story about a martyr. It's a story about how people become martyrs and, if they're good guys, why martyrs are heroes.
*Oblivion's drones quite resemble machines from The Matrix; the film's use of cloning was stolen from Moon; Tom Cruise's pastoral getaway on Earth reminds strongly of the pre-mission setting in Solaris; rebel heroes wear black, villains wear white, a la Star Wars; the movie's climax is taken from "Battlestar Galactica," or, more likely, the climax of Independence Day, or, still more likely, the destruction of the Death Star; etc, etc, etc.