Note: I'm not considering what Summer Glau's character is to be a spoiler. If you don't know that, then you've probably never seen an ad or promo picture. Or, for that matter, heard anyone talk about the series, and they also made it clear what role she would at least claim to take. Also, the only thing beyond that that I'm going in spoiled for is the identity of a mid-season character. And all I know is their identity. No spoilers, please. (It's 9 eps, you won't have to hold it in for long.)
Uhm, I'm going to assume everyone is at least passingly familiar with the first two
Terminator movies (we and the show both ignore the third.) If not, here's what happens.
Sarah Connor Chronicles starts off in 1999. Sarah has put her mind back together as much as one can after
Judgement Day, and, though she and John are still in hiding, has even found herself a fiance. She knows, however, that it still isn't safe, so she packs John up and moves on, ending up in New Mexico. No sooner are they settled in, though, than they're found by a terminator. Fortunately, John's cute new classmate, Cameron, is a terminator who (so she claims) was sent back by his future self to save him, saying that someone else eventually creates the machines.
You know, John Connor has to be the only guy in the world for whom a slim young woman as the badass protector makes much more sense than the muscular male. I mean, let's face it, according to his parentage and childhood, men who go into battle die. Women who go into battle walk away when it's over. Also, for him, yelling "Mommy!" when in trouble isn't cowardice, it's common sense.
While
Judgement Day (and the original
Terminator itself) focused on what it's like to know your child will grow up hunted because he'll eventually save humanity from before he was born, neither really has the time to show what that's like when people aren't trying to kill you at the moment. Even before people are trying to kill them in
Judgement Day, Sarah's mental state then is at least partly due to six months of people telling her she's insane while she knows that he's out there with no one protecting him, and I don't think there's a lot of difference. (This is why Sarah's together-ness here doesn't really bother me, though I suspect it might others: a lot of the extremes in her behavior struck me as being a result of her current situation.) As this aspect is the part of the movies that was always the most interesting to me (they become rather generic once you remove their being about the hero's mother who has to prepare him) I highly approve of it's being one of the main focuses of the series.