BtVS by Whedon and Mutant Enemy. The opening situation is very loosely based on an X-Files episode by Chris Carter. Just using it as a reason to end school early - more a red herring than anything.
Takes place following 'Seeing Red', so ending arc of Season Six spoilers.
Dawn was sitting in chemistry lab when she heard a weird noise coming up from the drain in the sink next to her.
"Hey, can you hear that?" she asked.
"What? You can hear that from all the way across the room?"
"No. What? Oh, you mean it's over there too?"
Dawn grabbed a pair of tongs and stood up, warily eying the sink just as the first of the rats began to spill upwards into the room.
Chaos ensued along with a lot of high pitched screams as the teacher tried to organize an orderly evacuation before any of the rodents could manage to scrabble out of the sink basins and onto the tables.
Dawn was the third out and the first one to notice the noise of rat feet scratching on the other side of the closed door to the girl's bathroom. "Great." Quickly weighing her options, she pulled the fire alarm.
The evacuation closed the school for the day and so, after being one of the people questioned on the front steps by a red-haired woman and a black-haired man who claimed to work for the FBI, Dawn caught a ride to her house.
She opened the door and walked in, not seeing anyone.
She moved around the house for a bit until she felt someone staring at her and whirled around to see Willow staring at her.
"What happened?" Dawn asked, concerned, as Willow's blouse was spotted with blood. "Is every-"
"Buffy's been shot. I don't think she's going to make it and I don't have the mojo to help."
"Where is she? Who did it?"
Willow clenched her fists, nails pressing hard into her skin, at the memory of the man who'd been harassing them for months. "Warren. She's at the hospital, but I need you to come to the Magic Box with me, so we can figure out what to do. If I get enough energy I might be able to save her."
"Research party, eh?" asked Dawn as she followed Willow out the door.
"Something like that," stated Willow as her eyes flashed black. In her heightened emotional state, absorbing all the natural energy that she could, she could almost believe that she could see the green aura of energy that Dawn had been made from.
The monks' spell to hide the Key could not normally be seen through except by those with a reptilian nature or those rendered, however temporarily, insane. Tara had been able to see through Dawn before her mind had been restored the year before and... now...
Willow was sure she was going to make Warren pay.
The bell rang at the Magic Box and Anya turned to greet her newest customers, only to realize with a frown that they were Willow and Dawn.
Willow raised her hand. "I don't have time to talk." Pulsing her magic, she flung Anya across the room and into unconsciousness.
"Hey," said Dawn as she rushed over to make sure Anya was otherwise okay. "You can't do things like that."
"I needed to," Willow said in a way that made Dawn study her face carefully.
"What aren't you telling me?"
"Tara's dead. He fired two shots and she was by the window. There was nothing the ambulance could do to help, so there was no point in telling them what had happened. They had enough on their minds already."
"That's horrible. Can I do anything to help?"
"I can use you as a conduit to absorb the energy flowing through these books. It will leave them blank and cost the place some money, but I frankly think it's worth it to heal Buffy and kill the bastard."
"Good." Dawn stepped forward to shake Willow's hand. "I'm in. Hey. Too tight," she said, trying to pull away. "Let go. Your nails are digging into my flesh."
"Good," Willow said as a wind whipped through the room. Shoving her free hand into the pile of books on the table, the witch felt her eyes turn pitch black. She raised her head to the ceiling in a scream as she forced the stored energy from the lore books through her body and into Dawn's core.
Magics mixed wildly as the protections that kept the Key in a human form flowed and twisted, disintegrating Dawn into a green mist which Willow happily drank.
Suddenly except for Anya, Willow was alone in the room.
Laughter wrenched itself from Willow's throat as she realized she could no longer feel her legs.
Putting her hands down for balance, she swayed in a loose circle.
She grasped hold of the table as hard as she could, as if that would stop the world from swirling around her.
"Oh," said Dawn as she looked down along her re-formed body and realized that, despite the poor fit, she was wearing Willow's clothes. "Crap."