The small town here is Chester's Mill in Western Main - just north of Castle Rock, where so many King stories have been set. One fine October 21st, an invisible barrier - not really a Dome, but that is what it will be called - drops down over the Mill (as its residents call it), cutting it off totally from the outside world. A very small trickle of fluids like air and water can come through, as can light and sound. Several people die immediately.
Our main more-or-less hero is Dale "Barbie" Barbara, an Iraq veteran and short-order chef, who's trying to leave the Mill when it comes down. because he is on the inside, he is reactivated, and promoted to Colonel, and told to take over the running of the town for the duration.
The problem with this is "Big Jim" Rennie, who, as second selectman, runs the town as his personal satrapy (the first selectman being a likeable tool), and he is having none of it. Rennie thinks of the Dome as a personal challenge and opportunity to protect the town while molding it to his liking. To this end he hires a lot more police - "for the duration."
Rennie's son Junior is a chip off the old block; the first thing we see him do is strangle a girl and hide the body, before he even knows that the Dome has fallen.
The Federal Government, quite plausibly, declares the area off limits to anyone except military personnel, headed by Barbie's old superior, a Colonel Cox. They cut off phone communications to and from the Mill, but leave the Internet in place pending certain conditions. President Obama - one of several real people who come into this novel - promises, the way Presidents do, that no step will be left untaken to free the people of Chester's Mill, and, indeed, the military makes strenuous (if not very rapid) efforts to do so.
A large part of the story is simply the conflict between Barbie and his allies, as underdogs, and Rennie and his allies. Bad things happen. Many people die, and there is a huge blowoff that is well rooted in the foregoing plot.
One problem with a novel like this is that the mystery of the Dome is far more satisfying than any possible explanation of where it came from and why. King does the best he can, and I doubt anyone else could do much better, but the secret of the Dome was bound to be a disappointment from the git-go.
(Indeed, several of King's major works - notably _The Stand_ and _The Dark Tower_ - are marred by disappointing denouements, despite their fascinating characters, extraordinary pacing, and compulsive readability...)
Well, it's a damn good read and was well worth the eleven days it took me to read it...