In his absence a monolith has exploded Jupiter, turning it and its moons into a secondary solar system. Ganymede, one of these moons, is colonized by earthlings, but another, Europa, is watched over by a monolith that, it's theorized, is monitoring human affairs and/or spurring the development of the plant-like beings who live under its frozen surface. The situation turns grave when Poole learns from his old colleague Bowman, who along with HAL has become a program running within the inscrutable monolith on Europa, that the monoliths' superiors may have soured on humans and be up to something nasty.
That's enough of the plot. It hangs together reasonably well, although in this third and presumably final sequel to 2001 Clarke has to struggle to pull all the threads together into a coherent narrative blanket. Much of the enjoyment of the book comes from other sources: the high-tech thingamajigs that often differ interestingly from their present-day analogues and the barely- disguised commentary on such contemporary issues as prison reform, Freudian therapy, clitoridectomy, terrorism, religious manias, and, of course, computer security and complexity.
Clarke was criticized for grossly underestimating the effort necessary to develop a computer with HAL's ability to converse (and lipread) by the year 2001. In 3001 his attitude to consciousness and artificial intelligence appears to have changed; there are marvels aplenty, but the only real agents seem to be people. Amazingly, even the monoliths are not seen as conscious.
The many social views implicit in the book are those of a sane, cultured man of the 20th century, those of, say, Mr. Clarke, whose interesting nonfiction addendum contains technical, historical, and personal notes that reveal the author's kindness and scientific knowledge. The hope, perhaps, is that such attitudes will become more dominant in the 31st century. As in much of science fiction, the psychology of the characters is serene, straightforward, and solitary. Unfortunately, this is not so for some of the series' aficionados who, with an air of overwrought erudition, have deconstructed and ascribed cosmic significance to virtually every aspect of the books. These fans will likely embrace the new book no matter what; happily, readers who prefer science and fiction to science fiction may also find it appealing, albeit not Jupiter-shattering. The problem is that even a monolith lacks the computing power to catch a breaking wave in the turbulent sea of cultural change. Doing so requires luck and timing. 2001 had them, while 3001, despite its straining for a similar numerological resonance, probably doesn't.