In the year 2270, with travel to the nearby planets well established, a bizarre discovery is made on Callisto, the eighth moon of Jupiter. Dozens upon dozens of strange wheeled artifacts--wheelers--are found buried beneath the icy surface. No one knows what they were used for and who left them in our solar system. At the same time, it is discovered that the moons of Jupiter have moved from their age-old positions. A quickly formed expedition finds that Jupiter is inhabited by a race of balloon-like aliens, who defend their world against comet strikes by moving their moons using gravitational technology. This time, though, their redirection is aiming an incoming comet directly at Earth! Communication at first proves impossible, but an Earth child who has an intuitive understanding of animal behavior becomes the key to contacting them--and joining forces with them to save the world.