Captain Peter Owensford of Falkenberg's Legion recounts how, as a Lieutenant in a Volunteer Brigade sponsored by the Earth Humanity League, he was part of an intervention on the side of Republican forces against the local rulers, a Spanish aristocracy known as Carlists. Thanks again to dumping of colonists from Earth, the Santiago colony on Thurstone has progressed to de facto slavery by debt bondage in order to maintain social order. Parallels with the Spanish Civil War are many and, according to the author, intentional.
The campaign is brutal, especially with a detached officer corps of political appointees unable to make proper military decisions. A "political officer" in the zampolit Soviet style, parroting liberty and atheism instead of communism, overrules Owensford's tactical decisions and impedes his training of the men. In the end, after losing many troops, Owensford catches the officer and some cronies preparing to use an atomic bomb to destroy his command and provoke the CoDominium Navy to act against the Carlists. He confiscates the bomb and attempts to retreat; but is caught in a Carlist advance. Eventually he surrenders to mercenary forces (who as detached professionals are less likely to execute him out of hand), and becomes a mercenary himself. His commanding officer in the mission, Captain Anselm "Ace" Barton, has already done so.