The characters in the opera The Visitors are escaping the Black Death by sheltering in a villa outside what city
The Visitors (opera):  The libretto (like those for "Pagliacci" and "Ariadne auf Naxos") uses the device of a play within a play to reflect and intensify the relationships between the protagonists, who in this case are loosely based on characters in "The Decameron". The Decameron:  The book is structured as a frame story containing 100 tales told by a group of seven young women and three young men sheltering in a secluded villa just outside Florence to escape the Black Death, which was afflicting the city.
Florence