It’s a slice of life series. For what its main focus is, it’s decent enough. It’s the “two-layer rule.” Why don’t vampires travel in the sewers? Sewer alligators. Why are there sewer alligators? Sewer alligator-taming vampire-hunting nuns. Why are the nuns there? To keep vampires from using the sewers. You answer two questions, and then loop it back into the main idea. It reinforces the main theme, and deters all but the most pedantic fans from pushing further.