When the Inter-Planetary Corporation's crack liner *Arcturus* took off on a routine flight to Mars, it turned out to be the beginning of a most unexpected and long voyage. Attacked by a mysterious spaceship, the liner crash-landed on Ganymede. The survivors first had to master that world's primeval terrors, then construct a new spacecraft, and finally, find a way to deal with the warring intelligences of the Jovian system. Spacehounds can do all these things, given time, resources, and freedom from attack. But . . when will these things start to run out?