"Thanks for visiting the Mystery Shack! Tell your friends!"

The corners of not-Mabel's mouth began to ache as her brace-filled smile began to falter, drooping ever so slightly before perking right back up again through what was, at this point, sheer willpower. The middle-aged tourist was none the wiser as he replied with a friendly wave and a smile of his own before he exited the Shack and returned to his tour group waiting next to the bus. As soon as the door shut, not-Mabel let out an exhausted sigh and rested her head on the counter, muttering, "Finally. Just a few more hours until sundown. You can do this. But first... a little rest." She shut her eyes and let out a contented sigh as Soos and Melody walked arm in arm into the gift shop. "You feelin' better, honey?"

"Much better." Melody tapped not-Mabel on the shoulder and said, "I know it was super short notice, but the baby's been kicking hard today. Thanks a bunch for helping, Mabel."

"No problem," she replied. "You think you'd be able to take over for a while?"

"As long as my passenger's okay with it," Melody replied as she patted her pregnant belly. "And hopefully he will be."

"SWEET!" A shot of adrenaline coursed through not-Mabel's body as she leapt to her feet. "I've got some important Summerween business to attend to!" Soos opened his mouth to reply, but the doppelganger exited the Shack before he could get the words out. She breathed in the forest air and let out a contented sigh as she crossed the parking lot and headed towards the woods with a sinister grin on her face. However, the familiar sound of a diesel engine filled the air as the real Mabel forced her way back into consciousness. Her smug voice echoed in the imposter's ears as she snarked, "Uh-oh! Sounds like another busload of tourists! Better get back to the Shack and help!"

"Not happening! If I ever see that many simple-minded rubes in one place again, I'll jump off the railroad trestle!"

"You're in Gravity Falls! THEY'RE EVERYWHERE!" Mabel let out a maniacal laugh as the imposter grabbed at her head and ran into the woods to avoid detection. Once they were deep into the trees, not-Mabel's entire body began to spasm and twitch, a sensation that made the real Mabel stop laughing and exclaim, "Whoa! What the heck's going on?!" That question was answered when the imposter grabbed either side of Mabel's head and pulled up, removing it from its body with little resistance. It was at that moment the imposter began to shapeshift its body, becoming more alien and ancient as it undulated and transformed. It turned Mabel's head around to face the metamorphosis in all its disturbing glory as the creature's true form was revealed. What was once the body of a 13-year-old girl was now a hulking torso with long, jagged legs and equally long, jagged arms, and the index fingers on its hands were less fingers than they were massive knives. A deep red glow illuminated either side of Mabel's head as well as the underside of her neck as she screamed in fright, watching the creature form a new, crude, temporary head before it began to speak in a deep, ancient, irritated tone.

"ENOUGH! Your incessant jabbering is enough to drive a person insane! How on Earth does your brother put up with you?!"

"Like a champ," Mabel replied with a smile. "And if he's following the suggestions I left on that note, then he's already got the newspaper clipping about the headless bear! Going from that to a squirrel... man, talk about a downgrade!"

"It was the first living creature, animal or otherwise, to touch me on the Day of Transference, child. Though I cannot choose which form I will take on that day, I must take it regardless once that occurs."

"Day of Transformers? Say what?" Mabel's confusion was not lost on the creature, and it flashed a wicked smile on its temporary head as it began to explain. "Every summer solstice, I take the place of another living being. One touch..." It leaned in and touched Mabel's forehead to its own as it concluded, "...and the process begins."

"Wh... what process?"

"The one you are experiencing as we speak. When I take the liberty of removing my prey's head, the body and spirit remains... until sundown. And when that happens, the body and spirit die... and with your head on my shoulders, I live for another year until the cycle starts over again."

"How many times have you...?"

"Far too many to mention," the creature replied. "Countless forest animals, fish, birds, and the like have all fallen by my hands. And once the sun goes down... you will join them."

"But... but I don't wanna join them! I CAN'T join them! My family needs me! My brother needs me!"

"And they will have you... me... for another year," the creature retorted in an emotionless tone. "And once I make the trip back to Gravity Falls with your brother next year, the process will start anew... with him. And there is nothing that you can do about it." The creature's temporary head tilted back as it spoke until it severed with a sickening crack and fell to the forest floor, where it disintegrated into ash. Mabel could do nothing but watch in horror as the creature placed her head back atop its shoulders, transforming back into a perfect facsimile of her body, sweater and all.

"NO! YOU CAN'T DO THIS TO ME! YOU CAN'T DO THIS TO DIPPER! YOU CAN'T DO THIS TO MY FAMILY!"

"I already am," the creature sneered as it overtook Mabel's consciousness. "I suggest you sit back, relax, and enjoy your remaining hours. Once the sun sets... you will be me... and I will be you." Not-Mabel let out a grim chuckle and headed towards the Mystery Shack with an extra skip in her step to avoid suspicion. As soon as she was out of sight, Soos hit the stop button on his cell phone and saved the video recording. He sank to the ground with his back against the tree he had used to hide himself, stammering and sweating. His heart hammered in his chest and his fingers fumbled through the phone's icons in shock as he exclaimed, "Dude... I gotta warn Dipper!"

He tapped the Messages icon, picked Dipper from his contacts, posted the video, and hit send.

The blacktop of the state highway was within sight as Dipper, Wendy, Waddles, and Mabel's headless body emerged from the woods. The quartet were just about to step into the town proper when Dipper held up his hand and exclaimed, "Hold it!" Everyone stopped at his command as he grabbed a jack-o'-melon lining the side of the dirt road and handed it to Mabel. "It may be Summerween, but I don't wanna risk anyone in town seeing that this is real and freaking out. See if you can put that on as a – I can't believe I'm saying this – as a temporary head."

Mabel made the melon nod with a downward move of her hands before she raised it and placed it on top of her neck. Once the skin of the fruit contacted the glowing red stump below it, the pair fused together and became whole. Tilting from side to side, looking up and down, then looking left to right did nothing to dislodge the melon from its spot, and Mabel mimed wiping sweat from her brow as she gave her friends a thumbs-up. And as if the visual of a jack-o'-melon-headed human wasn't enough, she pulled the clipboard out of her sweater and wrote:

You can call me jack-o'-Mabel!

That slice of dark humor helped to ease the tension and caused Dipper and Wendy to start laughing, a welcome respite from the bizarre circumstances they were in. "Jack-o'-Mabel it is, then," her brother agreed as he wiped a tear of laughter from his eye. "We'll find that head thief and make it pay!"

"With minimal damage to your head, of course," Wendy added. Jack-o'-Mabel crossed her arms before writing, NO damage to my head plz.

"Minimize cranial damage. Got it," Dipper cracked. He had another joke ready to ease the tension and was just about to tell it when his cell phone let out its incoming text alert: the familiar "YEAH!" sound patch from Soos' keyboard. When Dipper opened the message and played the video, his blood ran cold as he saw not-Mabel remove her head and shapeshift into its true form. He was so shocked and disturbed by what he'd seen that the phone fell out of his shaking hands onto the ground, stammering wordless noises as Wendy picked it back up and dusted it off. She put her hand on Dipper's shoulder as Jack-o'-Mabel leaned in and gave him a hug, which did little to calm him as the video played on. She held his hand after breaking the embrace, and as the video played on, Dipper's demeanor changed. His hands became perfectly still, and his worried brow hardened into an angry glare as the creature's confident tone lit a fire in his soul. Soos had zoomed in at one point to get a better look at Mabel's head, and the expression of pure fear, disbelief, and terror on her face turned everyone's stomach as Dipper gritted his teeth and squeezed Jack-o'-Mabel's hand. While the melon's face remained static as ever, her grip matched his in intensity and she shook her free fist at the screen in a shared display of anger.

"This thing thinks it can take my sister's head – my sister's LIFE – without consequence?! Not on my watch! I feel like kicking some cryptid butt tonight! Who's with me?!"

Everyone, Waddles included, nodded in agreement at Dipper's oath. Mabel's free hand balled up into a fist once again before she pulled a black marker out of her sweater and drew a pair of angry eyebrows above the eyes of the melon, a move that led to an amused snort from Wendy and a determined smile from Dipper. Any good vibes that maneuver had generated were dashed a few seconds later when the creature announced its endgame:

"And once I make the trip back to Gravity Falls with your brother next year, the process will start anew... with him."

Time stood still as those words hit everyone, human and pig alike, like a freight train. When the video ended, Wendy handed Dipper's phone back to him, giving him the opportunity to tap the Photos icon. He cycled through the thousands of photos he'd taken, searching for the perfect motivator to counter the creature's declaration. When he reached the photos taken in February, one stood out the most: It had been taken just after Valentine's Day, and Mabel was wearing her familiar brace-filled smile as well as a bright red sweater with a dark red heart in the middle of it. Her face was squished against Dipper's, who wore an expression of surprised happiness as he reacted to his sister's hug. He selected the photo and turned his phone around so everyone could get a good look at it, and then he began to speak.

"Sundown's in a few hours, guys. If what that thing said is true, then we've only got one chance to save Mabel. But if there's anything I've learned after last year, it's that one chance is all we need. If the sun's gonna set on anybody, it'll be that thing. There's no way I'm spending only one more year with my sister, and there's for DAMN sure no way I'm gonna let it do the same to me."

Wendy's eyes widened at Dipper's curse as he continued. "Let's get back to the Shack and show this head thief what we're made of! Let's get in there and save Mabel, Soos, and Melody! And let's do it before Grunkle Stan and Grunkle Ford get back!" The universe had been listening to Dipper's rousing speech, and as soon as he mentioned Stan and Ford, his "Disco Girl" ringtone began to play as the screen faded from the picture of him and Mabel to an incoming call message: Stanford was calling via Facetime. His face fell as he saw the notification, and even more so when Wendy asked, "What do we tell 'em, dude?"

"I... I don't know," Dipper answered. "I say we tell them that this is just a temporary Summerween costume and leave it at that."

"Good idea." Dipper nodded and pressed the green Accept icon, allowing the dignified voice of Stanford Pines to ring out through the wilderness. "Hello, Dipper! We're making a quick stop for gas, and then we're back on the road for 50 more miles! How's everybody doing?"

"Oh, we're... doing just fine, Grunkle Ford! Just biding the time until you guys get back!" Dipper gave a shaky thumbs-up to Wendy, and she returned it with a nod as he said, "We're just taking a quick break from helping Soos out at the Shack, and..."

"You're sweating, Dipper."

"It's summer! Of course I'd be sweating! Why wouldn't I be sweating?" His nervous smile gave the game away as Stan leaned in and cracked, "That's liar sweat if I've ever seen it! Ya hidin' something from us, kid?"

"No! Of course not! Just our Summerween costumes! Mabel and I can't wait to show you!"

"Can you put her on? I wanna see that smilin' face of hers."

"That's... gonna be kinda hard, Stan," Wendy replied as Dipper turned his phone to face her. "We're almost to the main drag and she's back at the Shack."

"No, she's not," Stan said with a grin. "She's standin' right next to you! I'd recognize that sweater anywhere!" Dipper's eyes widened in fright as he was pushed out of frame by a certain green melon as Mabel gave Stan and Ford a wave hello. "Hey, there's my favorite grandniece," Stan said as Ford leaned in alongside him. "Looks like you costumed up early!"

"AAAAND that's all we can show you," Dipper exclaimed as he began to push Mabel out of the frame. "If she shows you anything more, it won't be a surprise!" Even though Dipper had the best of intentions, Mabel's sibling instincts kicked in and she began to push back, fighting against her brother's panicked shoves with all her might. She strained against his efforts to push her offscreen and forced her way back into frame, much to the amused enjoyment of their great-uncles. "Remind you of anything, Poindexter?"

"All too well, Stanley," Ford replied with a knowing chuckle.

"Mabel, come on! Get back! We can't... let them worry!" Dipper's nervous plea led Stan and Ford to exchange concerned glances as the latter asked, "Worry about what? Is everything okay over there?"

"Fine! Everything's-" Mabel's effort to once again be visible proved successful as she lowered her shoulder and shoved her way back into the sight of the phone's camera. The maneuver threw Dipper off balance, and as he scrambled to find something to cling onto, he grabbed the jack-o'-melon with his right hand and fell to the ground, pitching his phone towards Wendy as he hit the dirt. Despite his errant toss, Wendy adjusted to the device's aerial path and caught it with ease, pointing the camera back towards the twins. When the pair were once again in focus, she let out a shocked gasp enhanced by a pair of screams from the other end of the line. The noise jarred Dipper back to attention as he realized he was holding the melon in his hands, much to his horror. Panic began to set in as his attention was drawn to his sister's headless body, Wendy filming his sister's headless body with his cell phone, and the horrified, shocked exclamations coming from his grunkles.

"OH MY GOD! OH MY GOD!"

"How is that even possible?!"

"WHERE'S HER HEAD, FORD? WHERE'S HER HEAD?!"

"I... I don't know, Stanley! Just calm down and-"

"HOW THE HELL CAN I CALM DOWN? IN CASE YOU HAVEN'T NOTICED, OUR GRANDNIECE IS MISSING HER HEAD!"

"I KNOW! And there must be a logical explanation as to why!"

"THERE AIN'T NO LOGIC TO BE HAD, SIXER! HER HEAD'S GONE AND SHE'S STILL MOVIN' AROUND! HOW IS THIS EVEN HAPPENING?!"

"I can show you. Soos sent me a video." Dipper's declaration brought a halt to Stan and Ford's collective panic. Stan took several deep breaths while averting his eyes from the screen, while Ford became laser-focused on the screen in front of him. "Send it. Now," Ford demanded in a cold, driven voice. Dipper did as he was told, and twenty seconds later, Stan and Ford were watching the video on the latter's cell phone. Silence ensued as Stan's initial shock and panic began to harden into a righteous rage as he watched Soos' video. Dipper and Wendy could see his eyes blazing and nostrils flaring as he got angrier with every passing second, while Ford's face remained stoic despite a similarly blazing pair of eyes. When Soos' video finished, an uncomfortable silence hung in the air on both sides of the conversation before Ford broke it by saying, "I know what that creature is."

"You do?" Dipper, Wendy, and Stan's simultaneous exclamations were followed by another, more pressing question from the lumberjill: "How do we stop it?"

"I'll answer your questions in order. First, I encountered that creature during one of the first summers I spent in Gravity Falls, and once again the next summer, and the summer after that, and so on. For ease of identification, I took the liberty of naming it the 'Be-Head-er,' with hyphens separating the 'be' and 'er' from the 'head.'"

"That sounds like something I would've come up with," Stan chuckled to try and ease the tension. "Apple doesn't fall far from the tree, does it?" Ford gave his twin a quick side-eye and a knowing grin before he continued: "As for your second question... I don't know. The best guess I have is this: It needs to complete the transfer by sundown. That doesn't happen for another three and a half hours, so you still have time."

"But only just," Dipper mused as he glanced at his sister.

"Don't talk like that," Stan replied. "After everything you and Mabel pulled off last summer, this Be-Head-er thing should be no problem. Like Ford said, we're about 50 miles away, and we're gonna get there as fast as we can. In the meantime, get a few good punches in on that cryptid creep for your ol' Grunkle Stan, and get Wendy and Soos to help!" The lumberjill pounded her fist into her palm and exclaimed, "Can't wait!"

"And neither can I," Dipper added. "There's only one way this is going to end, and it's with my sister's head back on her shoulders! But in the meantime..." He grabbed the still intact jack-o'-melon and placed it back atop Mabel's neck, where it once again fused to her body. She gave Dipper an appreciative hug before she pointed towards the Shack and made a "let's go" motion with her arm, jogging in place as she shadow boxed the air.

"That's my girl," Stan said with a smile. "Go get 'em!"

"Let's do it, guys!" Dipper ended the call and began to run back towards the Mystery Shack with his sister by his side, and Wendy and Waddles behind them. The building was within sight as Mabel broke ahead of Dipper, her fists clenched and her stride purposeful. Four steps later, however, she clutched at her chest and fell to her knees once again, shaking her temporary head back and forth. This time the sensation lasted for a full minute before her strength returned, and when it did, she stood back up and voiced her thoughts with ink and paper.

NOT AGAIN! Ooh, this is SO frustrating! Let's kick some Be-Head-er butt!

"My thoughts exactly," Dipper replied as he tugged down on his toque.