Twenty-third-century civilisation is recovering from a decades-long anti-technology freeze that has left the world under-populated, the Moon and asteroids controlled by a Tibetan Zen Buddhist sect from a deep-space habitat, and interplanetary exploration in the hands of a few eccentric outcasts.
One such loner - Prudence Odingo - returns to Earth to report that she has recovered 100,00-year-old wheeled artefacts, from under the ice of Callisto, a moon of Jupiter. She is arrested, and about to be convicted on criminal fraud when the 'wheelers' abruptly come to life - and several of Jupiter's moons change their orbits, ready to propel a vast planet-destroying comet towards Earth.
The unimaginable and incredibly powerful creatures that live in Jupiter's hellish atmosphere have apparently declared war on humanity. Prudence must somehow discover why - with the help of her Zen Buddhist friends, and the archenemy pedant who once destroyed her career.