The story is set in a chaotic setting (it's a Rucker novel!) of an all-providing-but-oppressive society. The society is controlled in large parts by a supercomputer, PhizWhiz, and its political masters. But PhizWhiz is all algorithms and no spark of imagination, which leads to its one large goal - achieve a melding with a human mind to initiate the trait of personal creativity. The problem is that only a few human minds are capable of withstanding the information on-slaught a melding with PhizWhiz unleashes. Vernor Maxwell is one such genius.
Vernor is a Physics student who theorizes that the universe operates on a circular scale. That if you went small enough, you would ultimately become larger than the universe and a continuing "shrinking" will bring you back to your original size (shades of T-duality in String Theory and Escher's perpetual waterfall!). The novel is a description of how he, with the help of his friends and a mysterious professor (Kurtowski, author of "The Geometrodynamics of the Degenerate Tensor"), dupes PhizWhiz into helping him discover the circular scale in reality.
The description of the ultra-small regime as they shrink in size is very nicely done, though Rucker spoils it many times with needless and quite crude language, as is his hallmark. Mathematical references are spread throughout the book (analogies to flatland, the universe as an infinite-dimensional Hilbert space, etc). He does a very good job of explaining how such a state of affairs can arise, using the analogy of a continuously shrinking ring wound around the doughnut-hole of a doughnut. Excellent stuff there.
However, the idea that you can keep expanding and then shrink back to the original size to find yourself back on earth at the *same* place in the *same* time era is not explained in anyway except for plot convenience. At one point, the professor muses that perhaps the shrinking process is really a movement into the fourth dimension, which can explain how, from the perspective of the shrinking person, everything seems to be receding. That, I think, is a stark and unforced error since Rucker describes the ever-magnifying details of the surroundings as the protagonists shrink (e.g. the details of a cabbage patch at various magnifications), which implies an "in situ" shrinking, not a movement away from the three dimensions.
That said, it is an entertaining book and Rucker's books are always full of great ideas.