Cara didn't ask to be stationed right on top of the Hellmouth. She certainly didn't ask to put her extremely expensive education to use trying to convince a bunch of drooling American teenagers to pay attention to Latin verb conjugations. In fact, she'd strongly prefer to avoid demons, non-Slayer high school students, and other such unsavory creatures altogether. But, well, she's the one who decided to become a Watcher. She made her bed, she has to lie in it.
Of course, when she signed on, she didn't expect a missing Slayer.
The girl seems to have vanished off the face of the earth, which, of course, means demonic activity is skyrocketing. And, of course, Cara's the only one around with even the tiniest chance of keeping it in check. Which is why she's poking around in the boiler room during her lunch break instead of popping another aspirin and taking a nap on her desk.
Whoever this missing Slayer is, she'd better have a damn good explanation.
"I've been told it generally means 'kidnap and rehabilitate', but at this point I wouldn't be surprised if it meant 'kill' more often than they let on. As to what 'rehabilitate' entails, it varies based on the Slayer, but they'll use whatever they need to as leverage."
"...Oh, also, about that thing where they want to drug me and shut me up with a vampire. What do those drugs do to people who aren't the Slayer, if you know?"
"They're a mixture of muscle reactants and adrenaline suppressors powerful enough to take out the Slayer, so I'd assume they'd render an ordinary human ineffective and possibly immobile."
"I'm not sure how that would work, if they could only be used on Slayers. Probably something magical. Anyway, I thought you meant that we should steal them and use them on dangerous Council members, although that plan has some flaws."
"No, I meant that Rune could pass for me and has powers that don't depend on being able to hit things. Oh well."
"Yes, that would've solved that problem neatly, if Rune had had no objections. Which I suppose is not necessarily a safe assumption."
"Understood. Destiny, do we want to usurp anything else tonight or should I start dinner?"
"It's interesting," she says. "In most cases like this, vengeance demons create alternate timelines based on the wish and either pull people out of them or dump people into them. But your case doesn't feel like an alternate timeline, because there are differences that don't stem from my wish. So the evidence suggests that you were pulled from an already existing alternate universe."
"Not necessarily. It may be that in order to get a witch Bella you have to go through all the rigmarole associated with my world - the ended Slayer line, whatever explains my lack of magical immunity. That otherwise you just get a Slayer Bella, or at any rate a magically incapable one."
"That could be true. It still seems like it would be easier to create a timeline where, I don't know, innate mental defenses didn't exist and you stumbled upon magic by more conventional means."
"Then you'd get a witch-Slayer, maybe, which wouldn't have been much in the spirit of replacing her."