Contains a story by Terry Pratchett, around which Stewart and Cohen write about the Discworld.

In 2270, archaeologist Prudence Odingo returns to Earth with news of a bizarre discovery of hundreds of ancient wheeled artifacts beneath the icy surface of Callisto, the eighth moon of Jupiter. The wheelers come to life just as several of Jupiter's moons change orbit, causing a comet to hurtle toward Earth. An expedition reveals that Jupiter is inhabited by an alien race that has declared war on humanity. Now, Prudence must discover why.