With furtive references to Gnostic gospels and a cast of characters that includes a beautiful blond novice (with whom Luciano fancies himself in love), a corrupt council of cardinals, a syphilitic doge and the brutal Cappe Nere, or secret police, all abroad in Venice’s sinister maze of watery alleyways, the scene looks set for a cliffhanging thriller in the style of “The Da Vinci Code.”