Florence, 1423 - When a mysterious body turns up on the banks of the Arno in the seedy Ognissanti district, it is up to Mercurio Capolupo, investigator for the Bargello and officer of the Otto di Guardia e Balia, to determine the victim's identity and solve the riddle of its fate. What he uncovers is a conspiracy that could shake the Republic of Florence to its very foundations. Guardia is a tale that weaves from the palaces of the Florentine elite to the hidden cells of the criminal underbelly, from urban markets, taverns and brothels, through the forlorn recesses of ancient ruins and across the verdant Tuscan countryside.