Warning - Wikipedia contains spoilers
The book is set on a far future Earth where the uploading of mindstates into a world-spanning computer network (known as "the Crypt") is commonplace, allowing the dead to be easily reincarnated (though by custom, only a limited number of reincarnations are allowed). Humanity has lost much of its technological background, due partly to an exodus by much of the species, which left behind only those unwilling to leave Earth, and partly to the fact that those who remained (or at least their rulers) are fighting against the more advanced technology, like Artificial Intelligence. Meanwhile, the Solar System is drifting into an interstellar molecular cloud ("the Encroachment"), which will eventually dim the Sun's light sufficiently to end life on Earth. However, the Ancients have left behind a device (the "Fearsome Engine" of the title) to deal with the problem; the book follows four characters who become involved in the attempt to activate it, with the narrative moving between the four (who do not meet until very near the end) in a strict rotation.