An excellent start to one of the best historical mystery series around. Also very funny.
Marcus Didius Falco is an ex-legionary sent from comfortable Rome, in around AD 70, to the cold, wet, swamps of ancient Britain. He is an 'informer' or equivalent of private eye, and the emperor Vespasian needs a discreet enquiry made. A young lady of good birth also requires an escort - Helena Justina. She's a real headache despite her good looks and Falco would have dumped her in a ditch if it wasn't for her connections.
Silver pigs are ingots of silver from the mine workings in Britain, and some have gone missing - enough to bribe a section of the army who would be better not bribed. How do you vanish silver ingots? You replace them with lead. So Falco discovers, and he tries to track them down in the barely civilised territory under stern governors who actually leave all the work to young secretaries hoping to make a name for themselves.
Love it, and I recommend the whole series to you. Davis began alternating settings so one is set in Rome and another in the wilds of Judea or Germanica; very easy to learn history and geography by reading these fun adventures. The best of the most recent ones is 'Saturnalia'.