The series set in a steampunk/fantasy take on 19th century Paris, the city of flowers, where vampires and humans tensely coexist after a war between their species. the vampires come from another world, which overlaps with the human dimension at several borders, at least one of which is located in Paris. These borders were generated as an accidental result of an experiment known as “Babel”, and only vampires who were also created by the event can feel pass between worlds- though humans also seem to have little trouble ding so as long as they which a ride with someone who can cross on their own.
The blue moon is said to be a source of misfortune as well as being responsible for creating curse bearing vampires. Normally, vampires are born on red moons. The book of Vanitas was said to be created by a vengeful vampire named Vanitas that was made a pariah after being born during a blue moon. The book has the power to interfere with a vampire's true name, which causes vampires to become curse bearers, and loose themselves to bloodlust, after which they are normally beheaded.
The main protagonists are the vampire Noe, and the human Vanitas, the later of whom took his name after inheriting the legendary cursed book of Vanities from his master. Vanitas is a self proclaimed doctor that uses his namesake to restore vampires who have become curse-bearers to spite him.
As per the industrial inspired setting, the cast is fit in Victorian attire. Much of the official art incorporates, covers, and pages incorporate gear motifs, and the book of Vanitas is shown and described to be "a clockwork grimoire linked to a silver chain." In the very first chapter, Noe travels to Paris via the airship La Baleine. In chapter 5, we see the Parade of Charlatan in Amelia's memories, which includes miniature air balloons, some sort of musical cabinet/box organ walking on mechanical legs, an wagon-like automotive pumping steam/smoke out of large pipes, and illusions of turning gears amongst the fog. The Paris entrance to the vampire world, seen in chapter 6, is is a gate at the end of a hall lined with crisscrossing pipes. If anything, the vampire city, named as "Altus Paris", found on the other side is even more obviously steampunk than the human Paris.