Short version: Vampires have come to visit and they've decided to stick around for a while. What's more, they seem to have overcome most of those troublesome weaknesses that vampires usually have. It's up to a the witches to save the day!
The witches are back for their final outing. Since Granny's off in a cave somewhere, Agnes and Nanny Ogg go fetch Magrat. Tradition is tradition and Magrat is carrying around a little one, which means she must be the Mother. If Agnes is the Maiden–that must mean that Nanny Ogg has been relegated to the … Other One. A fact she is amusingly thrilled with.
There is a entire plotline about Agnes having essentially created a second personality in Perdita X that actually helps her resist the vampires. It's interesting but also a little weird.
I like the vampires. In normal Pratchett style, they're a nice combination of what you'd expect and a few new twists. The pixies/gnomes are strange. In the audio version, it's basically impossible to tell what they're saying–although that might be the point–so I just sort of tuned them out. I do hope/expect that some more of the denizens of Uberwald come back in some of the other storylines.
The priest of Om–and really the whole religion–is interesting. Imagine a church where schisms happen near weekly so that it's near impossible to know what's doctrine and what's blasphemy anymore.
A solid story. Not quite up to the level of Maskerade, but still well worth the read.