Inside the Shack, Ford made himself busy. Gas lamps were lit, the emergency generator booted up, windows and doors boarded shut - he was even plugging corks into the knotholes that peppered the building's outer fabric.

Pacifica and Dipper sat in the armchair with an ever-increasing level of confusion as the old man ran back and forth like an obsessive survivalist preparing for the fall of society.

"Grunkle Ford, what's going on?"

"Kids, there's a lot of weirdness in this town I've never had the chance to investigate. Some of that only comes about at certain times of year, certain anniversaries-"

"Like some...movie?" Pacifica asked, wrinkling her nose at the contrived nature of it all.

"Exactly." Ford replied, sternly. Seemingly unaware of the trope-laden nature of the anomaly. "And this particular one is particularly vicious. Destructive. All-encompassing. Vengeful."

Dipper's eyes widened in excitement. "Town ending?"

"I fear it could well be."

"What is it?"

"That's the thing. We only have anecdotal evidence. And it only takes place every fifty years."

"So…our last evidence is from 1963?" Pacifica cocked her head.

Ford huffed. "Yes. And people back then weren't particularly... reliable."

"Why?"

"You'll work it out in a few years." He said, waving away the question. "What we know is that it's destructive…"

The greying researcher in the trenchcoat peered gingerly through the curtains with his eyes narrowed, his silhouette casting in the pale moonlight and the touch of the lamps with certainty and purpose. The flicker of gaslight made his silhouette shift and quiver.

He was pausing for obvious effect. Though the man was the sort to refuse he was acting in a character archetype, he was eager for any opportunity to add theatrics.

"...and there was one thing in common with every written record."

Dipper held Pacifica's hand almost instinctively. "...What?"

"Beavers." The old man replied, loaded with disdain, fear - even a spark of paranoia.

Pacifica raised an eyebrow - and was even more surprised when she met Dipper's stony, serious gaze. She figured he was just mirroring the scientist he idolised most, but damned if it wasn't bizarre to see the town's buck-toothed wood gnawing pests be treated as a serious threat. "Beavers. As in the-"

"Kids, follow me - we need to get into the bunker. Get your sister, I'll get Stanley-"

"Wait, no, Ford, I really don't get it-" She tried to protest.

"You will." he replied, fiercely.

All at once, it began. The Shack was drenched in fog. The windows darkened, such was the sheer volume of mist and precipitation that smothered them. The gas lamps, one by one, ebbed away into nothing.

The temperature dropped sharply. To a sub zero extent. To the awe of Dipper, the fog began to freeze against the glass, flaking and splintering into paper-thin shards of ice. He could see his own breath, which was increasingly panicked and stunted.

The wind whistled, despite the world around them grinding into a deathly still, airless night. It felt, for but a moment, like all had come to an end, before the threat had even begun.

Pacifica and Dipper's eyes traced the perimeter of the building suspiciously, Ford holding his hands up for them to stay quiet as he caught onto a strange, distant squealing sound.

THUMP

WHUMP

BANG

If you were to ask Dipper, it was Pacifica who screamed in an embarrassing falsetto. If you were to ask Pacifica, it was Dipper. If you were to ask Ford, he'd tell you he couldn't care less. In any sense, they were panicked. The building shook and began to creak.

Ford backed away from the window in horror as a black, silhouetted creature suddenly forced itself against the glass with a horrific SLAM that echoed across the Shack, making its rotten wooden frame groan in agony.

This time, both of the kids screamed, and even Ford jolted back in horror at what faced them. It was a beaver, alright. Its eyes glowing a furious, fiery red that pierced through the darkness in thin, flaring beams that reached to the opposite wall like lasers.

Its bucked teeth were clearly on view, its whiskers flailing widely, its clawed hands scraping and scratching on the stained glass, furiously, its voice in a high pitched squeal that sounded less like a beaver and more like a thousand harpies screaming in Arabic.

Mabel came stumbling down the crooked stairs, with her bedsheets wrapped around her and a tired, dozy look of irritation on her face. "Wha's goin' on?"

"Evil beavers!" Dipper shouted back, leaping in front of Mabel to protect her.

"Cool. I had a fever dream about that once."

Dipper continued frantically trying to fight her back. "No, Mabel, this is serious! They aren't normal beavers!"

Mabel rubbed her eyes, still fairly disinterested and more frustrated by her interrupted sleep. "What do you mean? Rabies?"

"Revenge." Ford said to her. "Into the bunker. Move."

"Grunkle Foooord, can't you just make 'em go awaaaaaay?"

"If history can be trusted," Ford replied, leading them into the gift shop, "These things will be here for three or four nights-"

SLAM!

Another black, shadow laden creature slammed against the gift shop door, making the thing rattle in its frame - and ringing the little bell above it in an irritatingly chipper ding-a-ling.

It squealed furiously, and began tearing at the old door with equal erratic hatred and unbridled violence as its kin.

Mabel put her hands on her hips and glared at it. "We're closed, you big furry dummy!"

The creature only responded with even more screaming, its piercing red eyes illuminating the room like foglights.

"Man, these guys are rude!" Mabel said as she was hurried down the stairs into the laboratory.

"Four days of beaver vengeance, sweetheart. It happens. This is Gravity Falls." Ford said, pushing Dipper and Pacifica after her. "We're basically powerless until daytime."

"What, then these things just disappear for fifty years?" Pacifica asked.

"Correct." the old man replied, leading them downstairs.

"But this is a serious problem, right? Like, they must be going all over town…" Dipper said.

"There'll be carnage. I don't think anyone living in the town will really know what's going on." Ford huffed. "At the very least, most people will be asleep. It'll just end up being building damage, a few fallen trees…"

"This town is the worst." Pacifica huffed as she walked through obediently.

Dipper smirked. "And the best, right?"

Pacifica rolled her eyes and tapped his nose. "Dork."

The dim light of the generator's lighting below was warm and yellowed. The laboratory's vintage electronics, cables and office chairs created a strangely welcoming, dry environment compared to the frigid, dark moisture and screeching, angry giant rodents with an appetite for redwood, pine and human flesh.

Stan was already waiting with a face like fury. "Sixer, I want ridda these schmucks."

"How did you get down here before us?"

"Been here all night, Ford. One a'ya gizmos was bleepin' in my pocket." He said, holding up Ford's Vengeful Spirit Detector. (patent pending) "Figured you'd wanna know."

"Thank you, Stanley." Ford smiled as he took it. He barely recognised the huge grin on his brother's face from those simple words. "This is really useful. Answers a lot."

The device was the same size of an Apple Newton, which was apparently the last computer system Stan had learnt to use. It was every bit as clunky and difficult to control, and largely built using parts from the smartievision games console.

It was beeping away in a fairly innocent - albeit oddly catchy - tune that was pretty impossible to ignore, listing a series of numbers on its tiny LCD screen that apparently made sense to Ford and literally nobody else.

Thanks to the thing's ever increasing age, it was no wonder that he had to hold it up to just about every angle to make out the ghosted numeric code on its screen. He squinted and tapped it a few times to verify the reading. "Sagan's sake. This is terrible."

"What? What is it?" Dipper asked, peering over his Grunkle's arm. "Is it a Class 10?"

"Worse."

"Huh?"

"These aren't quite ghosts, Dipper. They're revenants. As solid as you and I, utterly vengeful. Angry."

"I thought they were beavers." Pacifica said, somewhat sarcastically - though tinged with a growing curiosity.

"Beaver revenants." Dipper said.

"Beavernants." Mabel gasped, hands on her cheeks. "Really rude beavernants! Who scratch windows!"

"And worse." Ford replied. "Much worse. We just have to hope the townsfolk are sensible, and spend their night asleep."

Stan rolled his eyes, more irritated by the night's events than surprised. "Wish we freakin' could."

Ford, rather than his eyes, rolled out sleeping bags from his survival store and smiled. "Well, we can try, eh?"

"Sleepover!" Mabel beamed. "I want the pink one!"

"Hey, no fair!" Pacifica laughed. "If she gets pink, I want the purple one!"

Ford chuckled as the somewhat foreboding atmosphere was soon masked by the group's natural rapport - and, armed with hot cocoa and marshmallows, even Dipper managed to succumb to it.

The young, beleaguered investigator sat down, took his cup and took a deep breath - then cuddled up with Pacifica and smiled, seemingly happy to forget the chaos and horror being wrought above at the furry paws of evil wood gnawing beasts.

For now, anyway.

They were, at least, happy to ignore the big issue and listen to Ford and Stan's exaggerated - and, often, worryingly unexaggerated - tales of exploration, investigation and derring-do. Even with the squeals, thumps, scratches and roars of the rapid rodents up above, it was a very pleasant evening.

Y'know, save for the fact they were in a big hole in the ground with a broken old portal looming over them.