If that sounds like an over-elaborate and implausible set up (how did the human race manage to build huge machines and initiate a deep-space colonisation programme after an apocalyptic war? why leave Earth in the first place? why Titan, of all places? why drain Earth's oceans for fusion fuel when most of the moons of Saturn are mostly water?), that's because it really is a set up. After rescuing the pilot of a crashed spacecraft (Olga Kurylenko), Jack begins to uncover the truth - which is, unfortunately, only slightly less implausible than the cover story, owes a big debt to Philip K. Dick and a bunch of SF films I won't mention because spoilers, and is full of the usual logic holes that allow for heroic gestures and explosions.
Still, the ruin porn of the devastated Earth is lovely to look at, especially on an IMAX screen, and while the story slowly unfolds you can pass the time spotting homages and allusions to Wall-E, Planet of the Apes, 2001: A Space Odyssey, and many others. And even though it devolves into a derivative, two-fisted actioner and gives neither Olga Kurylenko and Morgan Freeman enough to do, it is at least a widescreen SF film that is knowingly SF. What a shame that, like so many big budget SF shoot-em-ups, it lost its sense of humour somewhere in the production process.