Aliens/Huguenots/the Hapsburg Empire capture Paris (or some other place everyone happens to be in), and everyone has to live under occupation. Maybe some of them have the royal family and are trying to smuggle them out. Maybe some of them are pretending to serve the new overlords in order to spike them. Maybe some are building barricades and making life a pain in the ass for the occupying forces. Hopefully there's lots of angst about how various love interests are in TERRIBLE DANGER.