Sienna Barlow loves nothing more than riding around New Kansas on top of her mega-dog, Gus. He's one of the massive pooches protecting the human settlers of a strange planet. In fact, Sienna connects better with Gus than with other kids. So when a visiting official threatens to shut down the mega-dog program, Sienna sneaks off with her best friend. After she, Gus, and a stowaway crash their escape ship, they discover a danger to every human in their community - and launch a wild plan to save New Kansas._