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'm working on a project. It's not going well. I've been trying to fix something that a bunch of deeply idiotic people screwed up a long time ago, and I'm not making any significant progress even though I'm working my ass off and this is what I'm supposed to be good at, research and problem-solving and boring minutiae that everyone else is too lazy to pay attention to. Not that anyone else is helping, either. I want to punch everyone I talk to in the face sometimes, even my...assistant. Don't get me wrong, my assistant's great, she's worth twenty of your average high school student, but she can't help me with this. I just wish I had a witch instead of a Slayer to help me make some progress, or at least make it so I didn't have to do all these mind-numbing spells by myself." (Too late, she realizes she name-dropped the Slayer. She doesn't call attention to it and hopes her company didn't notice).
Cara pulls herself together. She has to fix this.
She starts to walk back to her apartment. On the way there, she calls Bella.
"You have reached the Slayer, please rate your emergency on a scale of one to apocalypse?"
"Good, you're alive. I don't think it's an apocalypse, but I was talking to a person and I mentioned you and the person turned out to be a demon. Or possibly an evil witch. It disappeared ominously shortly afterwards, so be careful. Make sure everything's all right with your family. If it was a demon, I have some suspicions as to what kind of demon it was, and I'm going to research them."
"Dad's across the room, I'll call my mom in the morning, she's bound to be asleep by now - what happened?"
Her walk did not take her very far. She enters her apartment building and gets into the elevator.
She fumbles around for her key for a bit, then enters her apartment
Pause. "I can still walk across the room without tripping. I'm gonna set down the phone for a second and try something more elaborate." Clunk.
When the door opens she startles and falls on her ass.
"What are you and why do you look like Bella?"
"Stay down, or you won't like the consequences," she says. She picks up her phone.
"Bella? Are you still there?"
"Are you threatening - look, I didn't mean to be here, is this your place, I swear I have no idea how I wound up here, I wasn't even casting anything!"
(She hopes against hope that the Bella-shaped-entity doesn't call her bluff).
She is wearing jeans and a T-shirt, although not a T-shirt that the usual Bella owns.
"You're a witch and not a Slayer, aren't you?" she says. "Where did you come from?"
"I said something inadvisable to a demon, and the inadvisable thing I said seems to have given it some sort of power over me. Namely, the power to create a witch version of a friend of mine. You could be a perfectly benign witch version of her. You could also be a hostile entity who's also convincing facsimile of a benign witch version of her. Given the nature of the type of demon I suspect we're dealing with, I'm more inclined to believe the latter, but I'm not sure how to prove it either way."
"You made a wish around a vengeance demon? And it got you me? What the hells did you ask it for? Or is it something more obscure?"
"I didn't know it was a vengeance demon at the time. I still don't know it's a vengeance demon, but that seems the most likely. I wished that my friend could help me with something that required the help of someone magical, and I mentioned that I required the help of a witch, specifically, and not, say, a Slayer."
"I'm a witch, but right now I don't feel very helpful, you are not the most welcoming hostess I've ever met."