The year is 140 A.D. A peace of utter annihilation has descended on the Roman Empire after the crush of the Second Jewish Uprising, led by Simon whom Jewish sources call bar-Kochba and Greek ones, Barchochebas, a self-styled Messiah who had bound half the armed forces of emperor Hadrianus.
Antoninus Pius has succeeded aging Hadrianus three years ago. His current plans include to shift the Germanic border further into the Barbaricum, to include new territory into the empire and to create a new Limes as a manifestation of Roman power.
The graver dangers are lurking on his own side of the border, however. One of the first to meet the emerging peril is the kitchen-maiden Charis who has been forced to follow her new masters into the north. Contriving of ways to escape them and their attention, she gets enmeshed in the sinister plot set up by the cultists of the dragon goddess, known as the Lady of All - and discovers that her masters are part of it.
But the powers of Rome are aware of the danger and place their own strawman against the Lady of All: Restitutus, a local priest of a Celtic goddess who would have preferred to spend his remaining life in secluded retreat from the world. The only two things that can stir him out of his depressions are the view of the starry sky and little Valeriana, a toddler from the neighbourhood who keeps taking perturbations into his house.
In 161 A.D., only one device like the Antikythera Mechanism is still known to a few: the Opus Gemini, once forged by the astronomer Geminus on the island of Rhodus.
Now Princess Iulia Balbilla returns, the legitimate owner of the Opus Gemini. Exiled by emperor Antoninus Pius 20 years before, she has been biding her time in the border city of Samosata on the river Euphrates where once stood the throne of her royal ancestors, the last kings of Commagene. But Antoninus Pius is dead now and Roma has new lords: Marcus Aurelius, also called the Philosopher on the Throne, and his co-regent, Lucius Verus. For the first time, the Imperium is jointly ruled by two emperors. And at once they have to face a mighty foe: the king of the realm of Parthia has taken the opportunity to invade the Eastern provinces. As Princess Balbilla escapes from Samosata's plundering, she plots her return to the summit of power. For this aim, she needs the Opus Gemini.
On her behalf, her nephew sends two terrible agents for her lost heirloom: Sedigitus the Brute, a misshapen and taciturn gladiator, and Pernica the Swift, a misanthropic barbarian woman with a tendency to go berserk. Pernica has her own motivation to plot murder once they get to the house of Restitutus. For he is the last person who has reportedly seen the Opus Gemini.
But Restitutus has vanished without a trace half a year ago. Only his adopted grandson Adrianus finds the lost item without recognising it and decides to keep it for himself. Unfortunately, he has a witness: Valeriana, who recognises in the Opus Gemini a welcome tool to decide their persistent power-struggle once and for all in her favour.