On a primitive, arid planet, Harkaan is exiled from his tribe for killing men, only to discover that the killing spree, and his banishment, are part of a deliberately engineered Darwinian selection process, which culminates in his induction as a starship trooper in an interstellar war fought by humans on behalf of alien overlords, the Therem, against another group of aliens, the Yal. This is Pl...