When Crater captures an enemy Crowhopper, a genetically-manipulated being, he refuses to kill 'it'. 'It' is genetically more human than not and she calls herself Crescent. Crater takes her to Moontown as a prisoner of war but treats her kindly, unlike all of the other residents who are cruel.
When Crescent is imprisoned for a murder she didn't commit, Crater comes to her aid and the two become fugitives, escaping into the vast expanse of hostile terrain. Crater learns that it might be that the cause worth fighting for is right by his side.