The movie (aside from a quick prologue set in 2003) takes place in 2018, after a self-aware computer system named Skynet became self-aware, decided humanity was a threat, and launched a nuclear war to wipe us out. Previous Terminator films have centered around time-travel where Skynet, unable to stamp out the last gasps of human resistance led by John Connor, have sent cybernetic killing machines back to the past to terminate Connor's mother, Connor himself, even Connor's future generals. The resistance of the future manages to send back one of their fighters to as protection, Kyle Reese, who ends up being Connor's father in one of those time paradoxes it's better not to think too much about. This one, however, is mired squarely in one set of battles concerning a mystery figure named Marcus and the future father, Kyle.
And that's one of the failings – the film feels somehow small. The action's not, with spectacular set pieces of lil' water terminators, big Harvester terminators, flying hunter-killers, Voltron-esque motorcycle...inators, etc. McG shows a far steadier hand putting these scenes together with more skill and seriousness than one would've guessed from the director of Charlie's Angels and Full Throttle. But every other installment has been an ultimate battle to preserve the future, and after the bombs fall and there's not much left to save, the conflicts in this one seem a bit flat. The main mystery of the movie is of new character Marcus Wright's identity, and once revealed, proves to be more of a head-scratcher than a fist-pumper. I'll save you from spoilers, but overall I think his contribution to the continuity is questionable at best.
The strong cast helps: Christian Bale brings suitable gravitas and growliness to John Connor, Anton Yelchin (Star Trek's Chekov, having a pretty big summer) is effective as a fresh-faced resistance fighter, Sam Worthington succeeds to whatever point possible as Marcus, altho Bryce Dallas Howard barely registers as Connor's wife Kate. But they're put through pretty familiar paces – while some of the callbacks to the first films are fun, even the initially intriguing desert setting eventually culminates in yet another fight in a factory.
A friend of mine reviewed the film as "too much action," and while I don't quite agree with that, I will say there's definitely "not enough of anything else." Apparently 40ish minutes of character development got left on the cutting room floor, so while I have hope for a more coherent director's cut DVD, the final theatrical film is a a string of set pieces and a jumble of unexplained ideas. Characters fall in and out of love with little explanation, alliances are made and broken without much reason, and what should be a major character's transformative moment raises more questions than it answers.
That said, the scale of the science-fiction action does carry the day... but barely. It does an adequate job setting up this next trilogy (it's not a Phantom Menace-level disaster by any means) but it's not truly a "salvation" of a somewhat fading franchise. But as they say in these movies – the future's not set; there is no fate but what we make. So here's hoping they make a more emotionally powerful Terminator 5, because despite this one's faults... I'll be back.