He is accompanied by his protégé Gaius, a dashing young cavalry officer with an unfortunate tendency to act without thinking, and an odd figure named Calvus, tall, slim, bald, incredibly strong, and rather clueless.

At the beginning of the story Aulus' own mentor, head of the Bureau of Imperial Affairs, has summoned him back to Rome on a matter urgent enough to pull him out of a deep-cover operation in Palmyra just as it was coming to its critical phase.

Invoking the authority of the Bureau, Aulus orders an ancient Liburnian out of dry-dock storage to be outfitted for a trip to Cilicia in southern Anatolia, technically a part of the Roman Empire but at that time under the control of Odenath, to provide some unspecified service for Calvus.

(In a sea battle with pirates, the Romans face a desperate plight due to the authorities in Rome having failed to provide an adequate Marine contingent; capture by brutal Gothic and Herulian raiders; the apparent warm hospitality offered by an isolated Christian community turns out to mask a virulently bloodthirsty, fanatic sect; a giant, predatory Allosaurus, displaced from the distant past, rampages in the countryside...) Calvus, though taken for a male, is actually a female (of sorts) sent back from the far distant future to destroy the seed creche or "brood chamber" of monstrous aliens—intelligent social insects—who, by her time, have multiplied into billions, emerged from their underground creches, and are in the process of destroying the human race.

(The book hints that the future human race has evolved to be somewhat different from ourselves, and that the six sterile sisters—akin to worker ants or bees—are uncommon only in their preparation for this particular mission; there is a suggestion that, waging an existential struggle against social insects, future humanity was driven to emulate these enemies...) Calvus is the only one of the siblings to arrive at the proper time in the past; there are strong hints that the presence of a handful of extinct creatures such as dinosaurs and sabre-toothed cats in Aulus' time are a side effect of the unintentional displacement of the other siblings into the far distant past.

The book to some extent explores the themes of social order, the client-patron relationship, gender roles, and time travel, but for the most part is a tale of action and adventure rather than a vehicle for philosophical musings.