The year is 1546 and a strange child has been abandoned at the San Barnabo shelter for foundlings in Venice. Sister Clara feels mysteriously drawn to him and is compelled to claim him as her own. She is the first of many for whom Marin will assume the shape of their lost or unfulfilled love.

As the child grows into adulthood the waters of the lagoon rise, fish appear on the streets and the city is struck by a plague with symptoms that resemble drowning. The people are desperate for salvation and find it in San Barnabo, sacrificed hundreds of years before in order to save the city, and now returned in the shape of Marin.