Ryana and Saline come from Paris. They got bored from torturing puny humans so they've come to Sunnydale to terrorize the slayer and her friends. Ryana and Saline own the Bronze. While Ryana takes cares of business, Saline started to raise zombies to terrorize citizens and the Scooby Gang. Ryana plans to capture Tara in order to get Willow. When she has Willow her human servant Katherine will corrupt her and then Ryana wants to turn Willow into a vampire. Saline wants to capture Riley for her own sick sadistic pleasures but the truth she's obsessed with Spike.
Zombie's aren't really Buffy's area so she calls Anita Blake for help. A few of Anita's companions follow her of course, for one reason or another. It seems that the vampires and zombies that Ryana and Saline keep sending after them are a little much so they call upon Angel for help. His crew tags along for the ride.
Mysteriously and it seems coincidentally Drusilla shows up not only to try to woo Spike one more time but to also cause trouble for our heros.