Mercenaries are gamblers - they fight for pay but they die without honor. To sell yourself as a soldier in someone else's war makes no sense unless your side wins. No one becomes a mercenary except desperate men, natural killers, and someone like Earl Dumarest. Earl sold himself because he had a quest and he always needed money to continue his search for the way home, home to Earth, a world whose existence nobody believed.
Earl's side lost and Earl, a survivor, had to pay the penalty - more service as a military target or a mission for a ruthless lady who recognized a man of heroic talents when she saw one at her mercy.
But there might be an unexpected bonus waiting for Earl at the end of that unwilling missions - the coordinates of a planet called Terra and just possibly the spectrum of a forgotten sun.