11 September 1683, Rome. Rome is a city on a knife-edge. The citizens wait anxiously for news of the outcome of the Battle of Vienna, as the Islamic forces of the Ottoman Empire lay siege to the defenders of Catholic Europe. Meanwhile a suspected outbreak of plague causes a famous Roman tavern to be placed under quarantine. Among this detained in the Locanda Donzello is the mysterious Atto Melani, a spy in the service of the French king. With the help of the young serving boy, he discovers a secret passage leading to a network of tunnels under the city. Their nocturnal journeys into the Roman underworld lead them to some startling discoveries about the deadly enmity between Pope Innocent XI and Louis XIV, and a plot to unleash a weapon of mass destruction in the battle between Islam and the West.