After saving the Venetian Republic in Ascension, reluctant spy Alvise Marangon returns in this second adventure, played out once again in the carnivalesque atmosphere of eighteenth-century Venice.

After Alvise is arrested in a tavern brawl, he is summoned to meet the Missier Grande, head of the city's powerful secret service. Rather than being expelled from the city, he is coerced into a top-secret investigation of the mysterious death of one of the service's agents. This death seems to be connected with a mysterious secret society - The Four Horsemen - whose roots go back to the fall of Constantinople, or perhaps even earlier.

But as he oversteps the bounds in his investigations, Alvise finds that he has not only The Four Horsemen against him but also the authorities of the Republic itself. And so he is forced to go on the run, becoming both the seeker and the sought, amid the tangle of Venice's back-streets and canals and the outlying islands of the lagoon.