Wendy pedaled her bike down the dirt road leading to the Mystery Shack, singing "Straight Blanchin'" to herself - and silently cursing the fact that she was doing so - as the foliage grew thicker and dust was kicked up by her tires. Some strategically placed jack-o'-melons lined either side of the road, adding a bit of Summerween flair to the proceedings as she continued pedaling in earnest. The Shack appeared up ahead, but she saw and heard trees rustling to her right, and there was no wind blowing.
"Must be an animal." Wendy brought her bike to a halt with a squeeze of the brakes and prepared to let it pass, but the animal turned out to be Waddles accompanying Dipper through the forest. She knew exactly where they were headed, so she secured her bike to a nearby tree with a lock and chain, swapped her helmet for Dipper's old trucker hat, and followed them towards the entrance to Ford's bunker. Wendy arrived on the scene just in time to see Dipper slap his forehead and exclaim, "Darn it! I forgot about the fake branch!"
"Need a hand?"
"Sure, Wendy." As soon as those words left his lips, Dipper's eyes widened in surprise, and he turned around to find himself face to face with his old friend. The pair shared a laugh before the lumberjill asked, "How are you guys doin' after last night?"
"Mabel's doing great, but I'm doing fantastic," Dipper replied. He pulled the note Mabel had written out of his vest pocket and showed it to Wendy, saying, "She made me a scavenger hunt for Summerween, and I'm here to find the next clue!" As she looked over the quintet of instructions, a deep sense of unease crept up on Wendy as she studied the handwriting. "Are you sure about that?"
"Huh? What do you mean?"
"I'm no penmanship expert, dude, but this looks like it was written in a hurry. If Mabel was gonna put a Summerween scavenger hunt together for you, she'd have that thing planned out at least a week beforehand. This probably took a minute for her to write. Two, tops."
"What do you mean?"
She handed the note back to Dipper and pointed at the lettering in CHECK THE STATUE. "Look at the E's. You see how the middle line is off in all of them?" As Dipper leaned in to get a closer look, not-Mabel stumbled upon the scene and took cover behind a tree, peeking around the right side of the trunk to listen and observe as Wendy's suspicion was proven right. Dipper's eyes widened in recognition as he said, "You're right. They're all crooked. The first two are pointing up, and the third one is pointing down. And not only that..." He tapped his finger on the first C in CHECK and added, "Look how thick that letter is compared to the others. The strokes are a lot heavier there than they are anywhere else. And not only that, but there's also a covered-up misspelling in STATUE. See the A?"
"Yeah," Wendy replied as she checked it out. "There was a T there at first, and the A got scrawled over. I'm telling you, dude, this isn't a scavenger hunt."
"If that's the case..." Dipper gulped back his nerves and said, "Let's check the bunker, though. Just to be sure."
"Got it." Wendy took off her belt and climbed the tree, activating the bunker's entrance via the fake branch. The tree began to sink into the ground as not-Mabel watched from afar with an intrigued grin on her face, and once Wendy, Dipper, and Waddles headed down the stairs to the bunker, she saw her chance to make a move. She grinned in anticipation and started for the bunker, but before she could get two steps in, she found herself being picked up and placed on the shoulders of Soos as he let out a friendly, "Gotcha, Mabel!"
"Oh! Soos! Y... yeah, you got me, all right," not-Mabel replied with a nervous laugh. "Say, how about you put me back down? I've got something I need to take care of."
"That's quite a coincidence, because I do too! There's a busload of tourists comin' through to start the day, and I need an extra hand just in case Melody gets tired, what with her carryin' a mini Soos and all."
"I'd be happy to help, Soos, but-"
"You would? Thanks, dude! Off to the Shack!"
"No, no, wait, wait, WAAAAAAAAIT!" Not-Mabel's cries went unheeded as Soos marched towards the Mystery Shack in earnest, and the doppelganger resigned herself to her fate. Her glum expression didn't last long, as it shifted to an expression of shock and horror as she watched the real Mabel's headless body emerge from the woods and approach the stairs to the bunker.
"HEY! Stop it right now! Don't you dare!"
Mabel's body responded to her double's angry cry with an invisible chin flick before heading down the stairs, leaving not-Mabel to stew in her own juices. She gritted her teeth and let out a frustrated groan as Soos asked, "What're you yelling at?"
"Nothing. Just a bear," not-Mabel replied as she crossed her arms and sulked. "Just a dumb ol' bear."
The shapeshifter was still frozen, still in the cryogenic chamber, and most unnerving of all, still Dipper. What was meant as a tool for intimidation was nothing more than water off a duck's back one year later as the real Dipper glared at the creature before quipping, "Yup, still creepy." Waddles let out an affirmative grunt as he sniffed around the shapeshifter's pod and the one next to it containing Mr. No-Face as Wendy asked, "So, what are you thinking?"
"I don't know," Dipper replied as he stroked his chin in thought. "If you're right about this not being a scavenger hunt – and it's starting to look like you are – then we're gonna need to find Mabel and see if she'll tell us anything."
"Sounds like a plan, dude. Let's get outta here." Dipper nodded and began to follow her towards the isolation room with Waddles bringing up the rear. It was at that moment that a familiar scent hit the pig's nostrils – the scent of grapes, glue guns, and glitter. He let out an excited snort and raced ahead of Dipper and Wendy into the room, where he found his master on her hands and knees rifling through an open cabinet. "Just who we wanted to see," Wendy said with a smile.
"I'm having a great time with your scavenger hunt, but we had some... questions..." The statement died on Dipper's lips as he approached his sister, who was still hard at work looking through the cabinet. He put his hand on her shoulder to get her attention, and that did the trick as she backed out of the small space and turned to face him. Or at least she would have if she still had a face.
And her head.
The sight sent Dipper into a horrified screaming fit as he fell backwards onto the floor, with Waddles letting out a shocked squeal and Wendy standing with her jaw and eyes agape at the sight before her. Mabel, for her part, had anticipated that reaction and moved in for a cooldown hug, wrapping her arms around her brother and holding him close as his screams gave way to panicked hyperventilation. Family Secrets had been the furthest thing from Dipper's mind all day, but now, it was all he could think about. He rambled like a man possessed as the movie replayed in his mind and his thoughts crashed into each other, exclaiming, "Oh my God... Mabel... how are you... WHY are you... HOW ARE YOU..."
Wendy nodded her head in agreement and exclaimed, "What he said!" Mabel broke the hug and put her index finger in the air in a "hold on a second" gesture before once again rooting around in the cabinet. Dipper and Wendy shared terrified, confused glances as Mabel's headless body found what it was looking for: a clipboard and several pages of blank paper. She pulled out a red pen from her sweater pocket and motioned for them to ask a question. Dipper obliged with a shocked, "Can you hear us? Can you see us? Do you know where we are?"
Mabel wrote down her answers and handed them to her brother: Yes, yes, in the bunker. That helped to calm Dipper's nerves ever so slightly until another question came to his mind: "Wait... how can you hear us and see us?" A few strokes of her pen later, Mabel had her answer.
With my ears and my eyes, silly!
A pair of frustrated dope slaps from Wendy and Dipper later, the lumberjill exclaimed, "Obviously, Mabel, but... you don't have a head! How is that even possible?!"
Magic! And I'm serious about that!
"This is nuts... this is absolutely nuts," Dipper said to himself as he paced back and forth with his hands on his head. The initial shock had worn off, but he could still feel his heart pounding in his chest, and he took some deep breaths to calm himself down. "What kind of magic? Who... what... did this to you?"
A head thief! After we went to sleep last night, they snuck into the attic and did this to me! They have my real head, but this is like a GHOST head! It's so weird! I can still hear you and see you, but I can't talk!
Wendy sat down beside Dipper and asked, "Well, if that's the case, let's test that theory one more time just to be absolutely sure." She held up the middle and index fingers on her left hand and asked, "Can you see how many fingers I'm holding up?" Mabel responded by holding up the middle and index fingers on her right hand, a gesture that reinforced the presence of her wits. "Okay, so you're still there, but not... there," Dipper said as he gestured at the empty space atop her shoulders. "I'm not gonna get used to that."
You won't have to, because I know you guys are gonna save me. If we could beat Bill, the head thief doesn't have a chance! I'm not worried at all!
She gave Dipper a friendly punch in the arm and was just about to do the same to Wendy; Mabel's fist was within a whisker of contact when she pitched forward and collapsed to the ground. She propped herself up with her left hand and clutched at her chest with her right, only to fall to the floor once again. Dipper and Wendy helped her to her feet while Waddles nuzzled Mabel's leg with his snout, a gesture she reciprocated with an appreciative scratch of the one spot on his back that the pig couldn't reach. Once the feeling subsided, Mabel put pen to paper and voiced her state of mind.
Okay... NOW I'm worried.