Story (Series, in this case): Adventures of a Line-Hopper (crossover with Buffy the Vampire Slayer)
Rating: Teen overall, though several stories are rated All Ages
Word Count: 263,876 and counting
Author's Summary: The Doctor met Buffy in an alternate timeline, one from before the War, in which Daleks invaded the gym at Hemery High School instead of vampires. Now Buffy and the Doctor must learn about this new reality.
Characters/Pairings: Doctor/Buffy-ish and yet basically gen with canon pairings, mostly 10th & 11th Doctor with references to 8th & 9th, Martha, Donna, River, Amy, Rory, and Buffy and the Scooby gang.
Recced because: One of the best things a crossover can do is give new insights into familiar characters as they interact with characters and settings that are canonically unfamiliar to them. This series does that well, as Buffy and the Scoobies interpret the Doctor in various incarnations through their own filters and vice versa. Also, the Doctor and Buffy's relationship is nearly as timey-wimey as the Doctor's relationship with River, not to mention the two intersect.
Perhaps it was because he was pleasantly talking to a vampire. Perhaps it was because that extra Slayer sense was going crazy inside her head. Or perhaps it was just because she knew that normal people from the 90's didn't wear brown pinstripe suits with red trainers. But one thing was clear: the moment she saw him, Buffy knew she had to kill him.
He was in the cemetery, at night — always a bad sign, in Buffy's book — in a dark, ill-lit mausoleum, chatting, rather amiably, with a vampire. Buffy was on patrol, as usual. The night was cold and biting, the leaves slick beneath her feet. Buffy gripped her stake a little more tightly, prowling through the fallen leaves and moist grass without a sound, as she snuck ever closer to them. She could see their faces illuminated by candlelight, their shadows dancing on the nearby walls.
"Well, yeah, I suppose you could do that," said the strangely-dressed man. British accent — bit more lively than Giles. What was up with the Brits and vampire-demons?
"Course, bit thick, isn't it?" continued the man. "Actively calling attention to yourself by butchering a bunch of humans — not really a smart move, in my book. They tend to get a bit upset about that."
"Not my problem," said the vampire.
The Slayer senses were really strong, now. Buffy had never felt them this strong before. It wasn't a tingle or an itch the way it usually was. No, this was like a light in her head had just gone, ding! And every step she took towards the pair, the feeling got stronger and stronger. Right, Buffy knew the drill. First, get rid of the vampire — might as well, while she was at it. Then came the witty one-liner. Then the slaying.
"Well, see, it sort of is, if you think about it," said the man. "Because those humans, when you get them angry, they can be pretty hard to stop. You ever been to a Wal-Mart on Black Friday? Blimey, worse than Salem, I'll tell you that."
The vampire scoffed.
Buffy was right outside the door. She was pretty sure neither the demon nor the vampire had heard her. She was about to go in for the kill, when suddenly, she paused, and looked around. That was weird. She could have sworn she'd seen something, out of the corner of her eye, but there was nothing. She concentrated, but beyond the chattering of the Britishy guy and the vampire, there was only the faint whistle of wind and… was that a soft shuffling sound? Or was that just the wind in the leaves?
Not important. Back to the vamp and the demon.
"Oh, come on," the strangely-dressed man was saying to the vampire, looking a bit more exasperated with every word. "I know you might not have full control of your anterior prefrontal cortex, but surely you must have something of a brain. Use it!"
"And why should I listen to a word you say, Time Lord?" snarled the vampire. "I know what your people will do to this planet if you report back to them. Why shouldn't I just suck you dry right now?"
The man's amiability washed away, suddenly, and there was a powerful anger radiating from him. "You know what I am," said the man. "Question is, do you know who I am?"
The vampire's eyes went wide, his hands trembling. "The Oncoming Storm…" he muttered.
"Oh, yes," said the man. "And let me tell you this. Every horror story you've heard, every nightmare you've had, every rumor you've been told about me… they're all true. Which means the question you have to ask yourself is this: is it worth it? Is it really worth it?"
The vampire was now truly terrified. Buffy was trying to jot down mental notes for when she told Giles about this the next day. She wasn't sure what kind of demon the man was (a Time What?) but with lines like that, he'd probably try to end the world by at least Thursday. Which was annoying, because Buffy had a test on Thursday, and she didn't really need the distraction.