OK, so Tom Cruise may have some wacky religious beliefs. OK, so he's had some problems maintaining a stable marriage. OK, so he turned Mission Impossible from a cat-and-mouse espionage thriller into yet another big dumb action franchise. OK, so he's not tall enough to play Jack Reacher. Quibbles aside, the guy chooses some pretty good movies, ones that probably wouldn't have gotten made without his star power behind them.
Here he plays Jack Harper, living in a remote tower high above a ruined Earth, with his lover/co-worker Vika. His job is to fix drones, which prowl the surface looking for "Scavs". The Scavs are the remaining aliens from the Earth-destroying war; they are trying to sabotage the terraforming of Titan, which requires most of Earth's water to be sucked up and transported off-world.
Or at least that's what Jack thinks is going on. That's not what's going on at all. It takes most of the rest of the movie to unwind the actual situation. (His and Vika's relevant memories have been wiped, so there's a clue right there.)
The true situation is revealed in such a way that it sort of makes sense, but I had a hard time trying to figure out the reasons for the deception. Maybe I missed something. Otherwise, it's got something for everyone: a twisty plot you have to actually think about, amazing special effects, intense action, romance, sacrifice... Good job, Tom.