Some centuries from now, the exploration and exploitation of the Solar System is in full swing. On the cold edge of the system, Bella Lind, captain of the huge commercial spacecraft Rockhopper IV, helps fuel this new gold rush by attaching mass-driver motors to organic-rich water-ice comets to move them back to the inner worlds. Her crew are tough, blue-collar miners, engineers and demolition experts. Around Saturn, something inexplicable happens; one of the moons leaves its orbit and accelerates out of the Solar System. The icy mantle peels away to reveal that it was never a moon in the first place, just a parked spacecraft, millions of years old, that has now decided to move on.