Stephen King is the kind of Mainer who’s not afraid to tell you exactly what’s on his mind. And what’s on the author’s mind, judging from last year’s stellar collection of short stories, Just After Sunset, and a brand-new novel, Under the Dome (Scribner, New York, New York; Hardcover; 1,088 pages; $35), is the state of America post-9/11, post-Iraq, and post-Bush/Cheney. King has taken a long, penetrating look, and what he sees has got him worried.

In Just After Sunset, King hopped from one location to another, changing voice and style and genre with the deftness of an old master. With Under the Dome he returns to his roots, both in substance and in setting. The new book, which clocks in at a magisterial 1,088 pages, is set wholly within the borders of Chester’s Mill, a fictional town of some two thousand souls in west-central Maine. The story, as King explains in an afterword, was begun in 1976 and then abandoned. “The manuscript was long lost,” he says, “but I remembered the opening section. . . well enough to recreate it from memory.”

You’d never guess that from the finished book, which feels both newly minted and timely — often painfully so. This is not a horror novel, though horrible things happen. It does not feel like a science fiction novel, either, though it rests upon one seemingly impossible premise: An invisible barrier has dropped around Chester’s Mill one bright October morning, which makes it impossible for anyone to enter or (more to the point) to escape. Airplanes cannot fly through this barrier — one slams into it in the opening scene, as if to prove the point — nor can birds, nor even the tiniest airborne particle, like ash from a woodstove. Rain does not fall through it from above, though a small amount of moisture can seep through. Gassy air molecules barely manage to penetrate, though they need a push.

The people of Chester’s Mill soon begin calling this unseen barrier the “Dome.” Its precise physical details — along with other, more mysterious features that appear as the story progresses — are crucial to the narrative, and King has evidently sweated to get them just right, relying in part (he tells us) on a longtime friend named Russ Dorr, with whom his collaboration goes all the way back to The Stand, an early novel which Under the Dome in some ways recalls.

Now you might think, at first blush, that if you were going to get stuck someplace indefinitely, a quiet Maine town wouldn’t be such a bad place for it. But the same might be said for the idyllic island setting of William Golding’s Lord of the Flies (to which, at one point, King specifically refers). The problem here is human nature, that perennially tricky and unfathomable factor that often proves stranger than the wildest flights of speculative fiction.

The chief fly in the ointment is a town selectman, used-car dealer, and God-fearing Republican called Big Jim Rennie. From the get-go we can tell this fellow is up to no good, though the extent of his badness takes a while to unfold. Big Jim is supported by a small army of dubious allies, from a wild-eyed meth cooker to a fundamentalist preacher and even to Rennie’s own sociopathic son. Arrayed against them is a hometown cast that is fairly Dickensian in its eccentricity: a former army captain embittered by his time in Iraq, an old-school Yankee newspaperwoman, a plucky physician’s assistant, the good old gal who runs the diner, the agnostic minister of the Congregational Church, and a trio of middle-schoolers armed with skateboards and adolescent wiles.

But that’s just scratching the surface. In the pages of Under the Dome we encounter the whole panoply of  contemporary Maine — family farmers and hard-pressed retailers, locals and “Massholes,” flinty old ladies, teenage stoners, just-plain folks who are neither heroic nor villainous but simply frightened and confused. Sixty-seven names appear on a list of characters at the start of the book — there’s also a map, which proves helpful — and by the end of the novel you feel you’ve gotten to know them all, and learned more of the dirty little secrets of Chester’s Mill than you really cared to know.

This is vintage King: exaggerated social realism attenuated by circumstances and plot-twists that run to the extreme  and served up in plainspoken, sometimes windy prose that includes such passages as this:

“Her upper body exits through the windshield, trailing intestines like party streamers, and splatters against the Dome like a juicy bug.”

This is not, as one of the chapter titles warns us, as bad as it gets. If you can handle it, Under the Dome is one hell of a read.