From Wikipedia: " Set in Rome during AD 72, the novel stars Marcus Didius Falco, an informer and imperial agent. The title refers to the law which stated that no Roman citizen who had been sentenced to death might be arrested, even after the verdict, until he has been given time to depart, the idea being that for a Roman citizen to choose exile outside the boundaries of the Empire would have been a fate worse than death itself.

Falco's closest friend, Petronius Longus, has finally caught one of the leading criminals in Rome, Balbinus Pius. But a quirk in Roman law allows a convicted felon, even a murderer, time to depart before the sentence is carried out. Balbinus' departure has left a vacuum in the underworld of Rome, and there is a crowd of criminals trying desperately to fill the void. Their first step is to engineer a robbery that reverberates throughout the city.

Falco is again called upon by the Emperor Vespasian to supply answers, as quietly and quickly as possible. A couple of murders, a kidnapping or two, and more suspects than Falco cares to count takes him, and his patrician girlfriend Helena Justina, to places a family shouldn't have to go."