The film–if we can even call it that–opens with a Tom Cruise monologue telling us everything about the backstory that a good movie would have let us figure out on our own. Some aliens came and blew up the moon, causing havoc on Earth, we won the war but lost the planet, blah blah blah. Then he mentions that he's had a mandatory memory wipe–which immediately raises a thousand red flags–before being dispatched to protect some machines that are converting Earth's water supply into fusion energy, because apparently human beings don't need water anymore.
Tom and his partner, an uppity redhead, will soon be heading back to Titan, one of Saturn's moons, to join the rest of the human race, and Tom is a little upset at the prospect of leaving his planet behind. Which is understandable, because the planet is still quite obviously more inhabitable than one of Saturn's moons.
The main protagonists of the film are the Scavengers; remaining members of the alien race that failed in their attempt to overthrow Earth. The Scavengers have been attacking the drones that Tom Cruise is supposed to be protecting, and Tom Cruise can't tell that they're just human beings wearing masks because he believes what Scientology tells him.
Some stuff happens, there are a bunch of over-the-top scenes with over-the-top music that are completely unearned, including a scene in the first twenty minutes involving some skinny dipping in an elaborate swimming pool. Epic swimming pool scenes have to come at least halfway through the movie, guys. See Wild Things or Showgirls as reference.
Anyways, Tom Cruise's wife falls out of the sky, and he can't remember her even though he can remember the final play of a football game that happened before the moon exploded. Him and his wife end up hanging out with Morgan Freeman, the leader of the Scavengers, and then everything else that happens is from either Moon, The Matrix, or Independence Day, reminding everyone in the theater that we should only see science fiction movies that get limited theatrical releases.