Julius Caesar, as we know, arrived in Gaul (now France) and announced "I Came, I Saw, I Conquered," but when Decius Metellus arrives from Rome, not seeking military glory but rather avoiding an enemy currently in power, he finds that although the general came and saw, so far, at least, he has far from conquered. The campaign seems at a standstill.
Decius's arrival disappoints the great Caesar as well. He has been waiting for promised reinforcements from Rome, an influx of soldiers to restart his invasion. Instead he is presented with one young man ridiculously decked out in military parade finery and short on military skills, accompanied not by eager troops but by one callow and reluctant slave, the feckless Hermes.
It soon develops, however, that Decius's arrival was fortuitous. When Vinius, the army's cruelest centurion (so-called because he commands a hundred soldiers), is found murdered, Caesar remembers that his new recruit has successfully come up with the culprit in a number of recent crimes. Murder is bad for morale, particularly since it seems quite clear that the murderer was one of Caesar's men. Caesar orders Decius to find the killer -- and quickly.
Although evidence points to the son of one of Decius's clients -- a youth who was the particular target of the centurion's brutality, Decius racks his brain to find a way to save him from the sentence of death. The investigation leads Decius to two German slaves of the dead man -- a dwarfish old man and a beautiful woman. They are puzzling; the man is arrogant, the woman haughty--very unlike slaves. There are unanswered questions. It soon becomes clear to Decius that only by finding and punishing the real murderer will it be possible to quiet the rising dissatisfaction with Caesar's unorthodox method of warfare and forestall a mutiny against the mighty Caesar's authority and aims.