The premise: a mysterious, indestructible dome engulfs a small town in Maine, trapping the residents inside with their sins. But really, the premise is never what makes King compelling so much as his characters and their choices. The hero of Under the Dome is an ex army officer so sickened by his combat service that he wants to be a drifter with no ties and no obligations. The villain is a sociopathic used car salesman who spouts scripture while committing unspeakable crimes, all in the interest of self preservation. He's willing to sacrifice an entire town full of people to protect himself from his richly deserved comeuppance, and our hero becomes increasingly determined to stop him. Along the way, a rich cast of supporting characters provide entertainment, pathos, and sometimes inspiration.
King's prose is smart, whip-quick, and lucid. He paints a picture with such sure brushstrokes that you can read an entire book of his without realizing that he is one of popular literature's great prose stylists. That's because like any good writer, he gets you to focus on the picture he's painting, the story he's telling, and not on how he's telling it. Still, I frequently find myself rereading his sentences, asking myself, "How did he do that?!" And being impressed. The guy uses words very well.
If you've got the time for a long, involving read, pick up Under the Dome. I'm surprised to find myself saying that I think it supplants The Stand as King's finest novel. Under the Dome will make you think about good and evil, how they often live side by side in the same skull, and how sometimes all it takes is the right combination of outside events to turn a peaceful town into a seething hotbed of panicked violence.
One caveat: If you find yourself asking whether a cute little town could turn this bad this quickly, I suggest you turn on the nightly news. It happens all the time.