It is AD 186, and Britain is the northernmost province of the hugely successful Roman Empire. In Glevum (modern Gloucester), Libertus, a freedman and pavement-maker, lives under the patronage of Marcus Septimus. When a body is found in the furnace room of a nearby villa, and identified as that of Crassus Germanicus, a retired centurion, Marcus asks for Libertus's help. A slave is missing and the solution to the mystery seems obvious. But Libertus soon discovers that Germanicus has many enemies, and he must use his mosaicist's skill to put together the pieces of a most deadly puzzle.