Venice in 1749 - the city has lost its political and financial primacy but has become Europe's pleasure capital, famous for its gambling dens, its courtesans, its hectic carnival, its music, art and theatre - and the most highly organised secret service in Europe. Alvise Marangon, born in Italy but brought up in London, returns to the city of his birth and quickly becomes involved in the protection of a visiting English lord and his beautiful sister. 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 from turning into bloody mayhem.