The True Threat - The Cull Chronicles, #3

When Earth is conquered by a technologically superior race called the Grath, humanity is forced to work--and fight--for them. But not all humans are content to submit to their masters... When we last saw Jason Cull and the crew of the Freedom, they had just succeeded in risking everything to trade one problem for another. The ruler of the mighty Bettarian empire had agreed to free the Earth from its enslavement, all Jason had to do in exchange was take care of the one thing in the universe that the emperor was afraid of.
Couldn't be that hard, right?