When a young tour guide, Alvise Marangon, offers to help an English Grand Tourist, little does he know that it will lead to his being embroiled in blackmail and conspiracy. To add to his woes, he is then forcibly recruited into the city's powerful secret service to investigate a murder case. A reluctant spy he may be , but he is a gifted one.

Amidst the world of gambling dens and courtesans, something momentous is being planned for the Feast of the Ascension, Venice's most important and spectacular holiday, and it seems that only Alvise can prevent the day turning into bloody mayhem.