Petronius Longus, captain of the Aventine watch and Falco's oldest friend, has finally nailed one of Rome's top criminals. Under Roman law, citizens are not imprisoned but are allowed "time to depart" into exile outside the Empire. One dark and gloomy dawn, Petro and Falco put the evil Balbinus aboard a ship.

But soon after, an outbreak of robbery and murder suggests a new criminal ring has moved into Balbinus's territory. Petro and Falco must descend into the underworld of Vespasian's Rome to investigate.