Disclaimer: Gravity Falls is property of Alex Hirsch and Disney. Writing and publishing this story does not involve any form of profit or monetary reward.

Knock, Knock

A chorus of crickets gently serenaded the dark forest that surrounded the Mystery Shack. Tucked in his bed, Dipper enjoyed the constant chirping as suitable background noise for the late hours of the night. He should have been sleeping by now, filling the room with another source of slow, even breaths, but his mind kept replaying the day's events. Given everything that had happened that summer, he should have been accustomed to the oddities that took over his daily activities, and most nights he could easily crash into his pillow and conk out for the night, but tonight he was wide awake.

Across the room, Mabel was out cold, haphazardly lying sideways on her bed with Waddles half on top of her. There was a slight feeling of envy at the way Mabel could always easily fall asleep, at least until he noticed the pig drool slowly starting to soak the side of her pink nightgown.

Wrinkling his nose, Dipper closed his eyes and exhaled slowly. Beneath him, the mattress gave a small creak as he relaxed his weight into it. Grunkle Stan really needed to invest in new mattresses if Mabel and he were going to return next summer, but Dipper had a sinking feeling that his request would be met with a flat no from the cheapskate, or perhaps he and Mabel would return to Gravity Falls to find purple-dyed mattresses that had been "borrowed" from the Laser Tag place.

And by "borrowed" he meant "not borrowed." Stolen. Taken under illegal circumstances.

The next time Dipper shifted and made the mattress creak in protest, he didn't think anything of it. After all, the sound had become second nature to the room, especially in the dead of night when he and Mabel were being particularly restless. Despite the annoyance he sometimes had with the abrupt or striking noises when he was attempting to lose himself in thought, he'd be lying if he said that Mabel's noisy antics or the room's typical sounds didn't help him to feel at ease. Hearing Mabel squeal about something or another while the television blasted some ridiculous commercial and the birds and bugs sang throughout the woods was what made the Mystery Shack feel like home. Absolute silence in this decrepit place would only send him into a fit of terror.

Not that he would tell any of them that, especially when Mabel borrowed the camera to shoot her weird segments. There was no need to encourage her.

Knock, knock.

Dipper's eyes snapped open. He hadn't even realized that he had almost fallen asleep until he was startled so badly by the sound of knocking. Sitting up in bed, he looked around the dark room, still finding Mabel sleeping away in the same position he had last seen her in. Waddles hadn't stirred either, but Dipper wasn't sure if pigs functioned like watchdogs or not.

The knocking sound had been distant, like it was on the front of the museum entrance's door. The only reason Dipper probably even heard it was because it was the dead of night, meaning the shack was completely silent, and the entrance was located right below their bedroom window.

He waited, not moving from his bed as the seconds ticked by. There was no telltale groan of a disgruntled Grunkle Stan getting up to answer the door, and the knocking didn't resume. Had Dipper been anyone else, he might have just assumed that it was his imagination, especially when considering he had been just about to fall into a deep sleep when he heard the knocking. Yet Dipper knew better than to pass his experiences up to the easily explained and most normal assumption. Someone had definitely knocked on the entrance door right below his bedroom window.

Determined, Dipper slid out of bed, slipping his shoes on that he had kicked off before crawling under the covers earlier that night. Quietly, he pressed himself against the small desk that sat right next to his bed and in front of the window. Using the table as support, he leaned over it and looked out the window at the ground below.

Nothing unusual stood out; not even the weird shadows he often saw dancing across the yard late at night. This, of course, didn't mean that there was no one there. The person could have easily been standing directly in front of the museum entrance, making it difficult for Dipper to crane his neck to see directly underneath the window.

A frown stretched across Dipper's lips as he stepped away from the window. He debated changing into his day clothes, but he really wasn't interested in making this incident a huge project. Despite how awake he felt now, he did want to get back to bed at some point.

For some time his gaze lingered on the journal that was perched safely on top of the wooden desk by his bed. Instead of grabbing it, Dipper reached for the flashlight that was left next to it from his usual nighttime reading. His instinct nudged him to also take the journal, but the last thing he wanted to do was have both of his hands full when confronting someone at this hour. He needed at least one hand to shoo them off with.

Giving one last glance to his sleeping sister, Dipper tiptoed out of their room, leaving the door open just a crack. The stairs leading up to their bedroom were enshrouded in darkness, forcing Dipper to blindly reach out to touch the wall in order to guide himself down. He tightly gripped the metal flashlight, refusing to turn it on in case it alerted the knocker to his presence. Once on the first floor, the moonlight dimly lit up the room, and Dipper slowly made his way to the museum.

The door to the entrance of the museum remained closed, and from a safe distance, Dipper peered through the window as he crept behind some phony display that vaguely resembled a huge faux fur coat hung up on a small, wooden rack. Maybe it was just a coat rack and not part of the displays, but this was Grunkle Stan and the naive tourists of Gravity Falls that he was talking about.

No matter how long he looked through the small, square window of the door, no figure or even a shadow appeared through it. Unless the person was shorter than the window, it appeared as though no one was there.

"Huh," Dipper breathed out, relaxing his stance. Gingerly, he crept across the museum floor, slowly approaching the door. When he was right in front of it, he looked through the screen of the window, hearing only a louder chorus of crickets outside the door. His hand steadily grabbed the knob, and before he knew it, he was opening the door.

Only the dark of night loomed in front of him. Turning on his flashlight, Dipper looked left and right, but he could clearly see even without the assisted light that there was no one there. With a soft click, Dipper shut his flashlight off and went back inside the Mystery Shack, closing the door behind him. Now he could label the noise as being some mundane occurrence. A raccoon or possum could have tapped against the wood while it scurried past. There might have even been a sudden wind that shook the door just enough to knock it back and forth since the wood didn't exactly fit inside the doorframe perfectly.

Shrugging it off, Dipper made much less of an attempt to remain quiet as he noisily yawned, stretching his arms over his head as he made his way back across the museum and headed towards the stairs. Taking a few steps up the rickety staircase, he already imagined his next move: Plopping down on his bed and curling up inside his blankets.

Knock, knock.

Dipper froze. His right foot hovered over the next step as he tightly clutched the flashlight in his hand. This time the knocking had come from the direction of the gift shop.

Backing down the steps much quieter than before, Dipper made his way through the house and towards the gift shop. Peeking underneath the curtain, he stared hard at the entrance to the gift shop, expecting to at least see some figure or shadow now.

Again, he couldn't see anything from his position. Maybe the person had tried the museum entrance and was now trying the gift shop?

Not wanting the person to wander off, Dipper pushed past the curtain and briskly walked through the dark gift shop. Leaning forward, he looked through the screen window, trying to make out anything.

"Hello?" Dipper greeted, still looking through the screen. "Is someone there? Hello?"

No answer. Dipper clicked his flashlight back on and opened the door, pointing the flashlight to his left and right and even around the yard. Once again, he was greeted with nothing.

"Okay..." Clicking the flashlight off again, Dipper stepped back inside the gift shop, shutting the door. "I know I heard someone knock, but—"

Knock, knock.

Dipper trailed off, snapping his attention towards the back of the house where the back entrance was. This time, Dipper didn't hesitate. Racing through the shack in the dark, he practically slammed into the back door as he quickly opened it, jumping out as though he was going to catch the knocker red-handed.

"Aha!"

Panting slightly from the rush of adrenaline, Dipper could only hear the sound of his breathing mixed with the crickets and such. An owl hooted from somewhere in the woods.

There was nothing there. No one. There was no feasible way for someone to run away and not make a sound or at least be out of sight by the time Dipper got there, unless it was their intention to knock and dash.

"Who's out there?" Dipper demanded to know as he allowed the back door to shut. His shoes softly plodded against the dew-covered grass as he began walking around the backyard. He pointed his flashlight ahead of him, clicking it back on so that the beam of light could penetrate farther into the yard. The light skimmed over the tree line as Dipper stepped farther away from the house, but even with all the creepy shadows the woods created, there was nothing that resembled an animal or a person or even a monstrous being.

Turning around, Dipper focused his attention on the shack itself. Perhaps the knocker was actually hiding around the corner? Pointing the flashlight in its direction, he inspected the back of the house, finding nothing.

"You've got to be kidding me!" Dipper groaned, jogging around the house and causing the light of his flashlight to bounce as he inspected the entire perimeter of it. He even stopped at one point and quickly ran in the other direction, thinking that if the person was always just ahead of him, he could catch him off guard by circling back. Again, Dipper saw nothing. "This is crazy!"

A bit agitated, Dipper shut the flashlight off and stormed back inside the house, only partially attempting to remain quiet as he quickly bounded up the stairs, making a beeline towards his room. Without thinking, he shut the bedroom door rather loudly, causing his dozing sister to visibly flinch.

"Wha?" Mabel groggily sat up, yawning and rubbing her eyes.

Guilt ebbed away at Dipper's stomach. "Sorry, Mabel. Go back to sleep."

"Dipper?" Mabel blinked some more sleep out of her eyes, sitting up some more so that Waddles slid off of her and onto the bed. "What's going on?"

"It's..." The word "nothing" died in Dipper's throat. He swallowed, gesturing to their bedroom window. "Someone was knocking on all of our doors, but when I went to go look, I couldn't find anyone."

Mabel closed her eyes, and for a second, Dipper thought his sister had fallen back asleep while sitting up. "It was probably just the wind."

"No." Dipper shook his head. "Someone or something knocked on the museum door, then the gift shop door, and then the back entrance."

"Just now?" Mabel asked to which Dipper nodded. Blearily, she opened her eyes again and peered out the window from her position on her bed. "At this hour?" She frowned. "Darn solicitors. I'm illegalizing them in the morning."

"Uh, wouldn't a solicitor want me to find him?" Crossing his arms over his chest, Dipper stalked over to his bed. "It's probably some stupid teenager—like Robbie."

Bringing her knees to her chest, Mabel let out another yawn. "I dunno if Robbie can run in those tight jeans." A wide grin adorned her face. "He'd have to waddle away like Waddles."

As Mabel adoringly petted her pet pig, Dipper sighed.

"Whatever it was," he said, kicking his shoes off. "It's gone now, so I'm—"

Knock, knock.

Both twins snapped their heads over at their bedroom door. Instead of feeling agitated or annoyed, Dipper's stomach dropped, and he quickly looked over at his sister with wide eyes. It was one thing for the knocking to occur on all the outside doors, but now the knocker was inside the house.

"Sorry, Grunkle Stan! We didn't mean to wake you!" Hopping off her bed, Mabel obliviously made her way to the door. Shock prevented Dipper from stopping her until it was too late.

"Mabel, don't!"

She opened the door.

Silence filled the room once more as an empty hallway greeted the twins. Mabel stuck her head outside their bedroom, tilting her head as she continued to find no one.

"Huh, that's weird," she commented, stepping back inside their bedroom.

Not wanting to chance it, Dipper hurriedly grabbed the door and shut it. He grabbed Mabel's arm and tugged her away from the door.

"That's what kept happening!" he hissed at her, looking around for something to use to protect them. Before he could head towards the closet to grab their golf clubs, the journal caught his attention. Letting go of Mabel's arm, he headed over to it. "There has to be something on this..." Flipping through the pages, Dipper stopped once his eyes skimmed over the word "knock."

Mabel peered over his shoulder as he silently read the entry. "The Knocker? That could mean something entirely different if it had an 'S' at the end—"

"Where's the black light?" Without waiting for Mabel's reply, Dipper turned and grabbed the device from the other side of the desk. Turning it on, he read the entry: "Warning to those who ignore the knock as the creature known as The Knocker will not stop until it is answered." Biting his lip, Dipper skimmed over the passage again. "This doesn't make any sense! It just says to answer the knock and the knocker will go away, but we've answered it four times now!"

Knock, knock.

"What do you want?" Dipper called out, slamming the journal shut. He glared at the door. "I've answered you, so go away!"

Knock, knock.

"Agh!" Dipper raked his fingers through his thick, brown hair. "Is it speaking in Morse code? Do I have to answer it in Morse code?"

"Ooh! I'll do it!" Mabel trotted over to the door and rapt her knuckles against it in some nonsensical pattern. A proud smile lit up her face when she finished.

Dipper stared at her. "Mabel, I don't think that was Morse code."

Knock, knock.

Putting her arms up, Mabel walked back over to Dipper. "I guess I don't speak knock, knock—wait!" Mabel's eyes got big as she pointed her finger up in the air. "That's it!"

Dipper looked up at the ceiling where Mabel was pointing. "What's it? I don't see anything."

"Knock, knock!" Mabel made a hand motion to Dipper, prompting him to give her something. He could only awkwardly stare at his twin, wondering if the lack of sleep was finally getting to her. When Dipper didn't respond, Mabel continued on. "Who's there?"

Realization unclouded Dipper's mind about what his sister was referencing. "I don't think the knocker is looking for a knock-knock joke."

Walking over to the door, Dipper hatched a new plan. He waited.

Knock—

Slamming the door open, Dipper was just about to announce to the knocker that he'd been caught, but his moment of eureka ended before it could begin. Holding the open door in his right hand, Dipper found himself face-to-face with...

A closed door?

"What the..." Dipper glanced from the open door to the closed one. His right hand carefully brushed his fingers over the wood, assuring him that he had opened the bedroom door and was currently holding it, but this didn't help to explain why he was facing another door.

"Wow!" Mabel's voice came from directly behind him. "How'd you do that?"

"I didn't. It was just," he gestured to the closed door, "there."

Mabel stood by his side, looking up and down the normal-looking wooden door. Despite mimicking their bedroom door, it appeared to open out instead of in. "Was this always here?"

"I don't think so," Dipper murmured. He eyed the closed door with suspicion. "Is this the knocker?"

"The knocker's a door?" Mabel giggled. "Let me try something!"

Without waiting for Dipper's approval or even explaining what she was about to do, Mabel leaned forward and knocked on the closed door. She stepped back in line with Dipper, looking expectantly at the door as though someone on the other side would answer it.

"Oh, no," Dipper said. "This could be bad—"

Knock.

That was it. A single knock greeted them. The twins stared at the closed door, waiting for something else to happen, but nothing did.

Before Mabel could volunteer herself, Dipper made the decision to try opening the closed door. His hand grasped the replicate knob of their bedroom door, and he attempted to turn it.

The knob wouldn't budge.

Grabbing the knob with both hands, Dipper attempted to turn it again. He pushed his bodyweight against the door, thinking that maybe it was stuck, but the door remained closed.

"It. Won't. Open." Dipper repeatedly slammed himself into the door, but it didn't even shake like their bedroom door would. Panic started to seep into his mind. "We're trapped!"

"Uh-oh." Mabel attempted to help Dipper push the door open. "This is a fire hazard!" She pointed at Waddles who had woken up from all the commotion. "Waddles, use tackle! Take down!"

Waddles oinked at her and refused to move from the bed.

"Oh, no! Waddles ignored his owner! I knew I should have made more badges!"

"Mabel, stop joking around!" Dipper grunted as his feet slid across the wooden floor. Maybe he should put his shoes back on for more traction.

"Oh, right!" Mabel stopped pushing her body against the closed door. "The knock-knock joke!"

Dipper groaned. "No, Mabel. That's not—"

"Knock, knock!"

Pinching the bridge of his nose, Dipper wondered why he bothered. There had to be some way to get this door to open. He would just have to figure it out.

Knock.

"Who's there?"

"It's a door, Mabel. It can't respond."

Knock, knock, knock.

Dipper stopped pushing against the door, shocked at the sudden change in pattern. Mabel grinned at her brother, continuing on with her joke.

"'A door' who?"

There was no knock this time—just the sound of Mabel giggling to herself, taking a moment to hold back the punch line.

"A-DOOR-ABLE!"

Mabel burst out laughing while Dipper leaned back against the closed door, putting his face in his hands. The last thing he wanted was for Mabel to spend the rest of the night telling lame knock-knock jokes.

"Mabel, can you please—Whoa!"

One moment the hard surface of the closed door was supporting Dipper as he leaned back against it, and the next, he was falling backwards right out of their bedroom and into the dark hallway. Arms flailing, Dipper grasped the doorframe at the last moment, propelling himself back into the room. He backed up and turned around, standing in front of Mabel as he inspected what happened to the closed door. Had it opened?

No, it was gone. Dipper blinked, rubbing his tired eyes. No, he was not seeing things. The door was there and now it was gone!

"Mabel was...right? We were supposed to answer the knocker with a knock-knock joke?"

"Woo!" Mabel cheered, throwing her arms up. "Ten points for Mabel!" She jumped on top of her bed, causing Waddles to bounce.

"Well," Dipper sighed for the umpteenth time that night. "At least it's gone."

Looking out their bedroom window, Dipper could still see the darkened sky and silhouettes of pine trees. There was plenty of time for him to sleep before the morning.

The thought of sleep after everything caused a smile to tug at Dipper's lips, and he yawned loudly as he made his way over to his own bed. Crawling beneath the covers, he plopped back on the bed, welcoming the loud creak from the mattress. Across the room, Mabel settled down with her triumphant boasting, throwing the covers back over her form as well. Waddles curled into her side, making a soft oinking noise before closing his beady eyes.

"Good night, Mabel," Dipper said, sharing his smile with her. Mabel gave him a toothy grin back.

"G'night, Dipper!"

Knock, knock.

Dipper closed his eyes and groaned. "Are you kidding me?"

"More jokes!" Mabel practically rolled out of bed, stumbling a bit since her sheet had somehow wrapped around her leg. She kicked the offending layer off and bounded over to the door. "Okay, okay. Here's one. Knock, knock!"

Mabel opened the door and promptly screamed.

At his sister's shriek of terror, Dipper also cried out in alarm and fell out of bed. While a mess of limbs on the floor, he looked beyond Mabel with the expectation of there being a monster that was the real knocker.

Grunkle Stan glared at them. "It's three in the mornin'! What in the blazes are you kids doing?"

"Knock-knock jokes!" Mabel answered having calmed down from her initial fright.

"Aye, yai, yai." Grunkle Stan pinched the bridge of his nose. "Knock-knock jokes don't literally involve all that knocking. I can't sleep with you knocking every few minutes on all the doors!"

"But Grunkle Stan," Dipper started to protest, but their Grunkle simply shook his head.

"Just go to sleep." Grunkle Stan turned to leave, but then he redirected his attention at the twins. "And tell me it's you two when I ask who it is. It's creepy when you don't say anything."

Grumbling to himself, Grunkle Stan made his way back down the staircase. Dipper and Mabel watched him go, giving each other an amused glance.

"Should we tell him about the knock-knock jokes?" Mabel asked, shutting their bedroom door once more and heading back to her bed.

Dipper shrugged. "Maybe in the morning. Let's just get some sleep."

Settling back in bed, the twins heard the unmistakable sound of the knocker downstairs, followed by a loud shout from Grunkle Stan. As Mabel giggled and Waddles oinked from all the commotion, Dipper lay back in bed, enjoying the typical sounds of an average night in the Mystery Shack.

Though those crickets were starting to get annoying.