Based on a dream I had.

It seemed like a typical day at Gravity Falls. Inside the Mystery Shack, Dipper and Mabel were restocking supplies, Wendy was slacking off, Soos being Soos, and Grunkle Stan charging exorbitant prices for his tours. Great Uncle Ford mostly stayed in the basement conducting research.
"Gee Dipper, I am so bored." said Mabel.
"Well Mabel... what do you want to do later?" asked Dipper.
"I don't know. Ugh! I am so bored." said Mabel looking out the window. She noticed someone stapling a flyer to a wooden post outside the shack. Seeing this, Mabel ran out of the shack to see what it was.
"Mabel where are you going?" asked Dipper. He ran after her.
"What is it?" asked Dipper.
"There's a fair tonight in town!" said Mabel with excitement. "Finally something interesting. Wanna come?"
"Uh... tonight?" asked Dipper.
"Yeah tonight why?" asked Mabel.
"Um... you know I'll think about it." said Dipper.
"Okay then." Mabel said with an eyebrow raised.

Later that night, Mabel was dressed in a festive sweater, and ready to go.
"Hey bro, ready for quality brother and sister night out?" asked Mabel.
"Oh shoot, that was tonight?" asked Dipper.
"Yeah, don't you remember our conversation earlier today?" said Mabel.
"Oh right. Um." said Dipper. "See here's the thing. I promised Great Uncle Ford I'd help out with researching the dimensional rift, and then later I agreed to help interpolate the-" Mabel interrupted Dipper mid sentence. "blah blah blah, nerd talk, and oh I'm a nerd who's too busy to go to a fun little fair with his own sister', that's all I hear." said an angry Mabel.
"Mabel, calm down. I promise as soon as I'm done I'll head on over and join you." Dipper assured.
"Well... fine. If you say so. I see you have your priorities all laid out." Mabel said stuffed up.
"Mabel are you crying?" asked Dipper.
sniff "No... I'm perfectly fine." said Mabel.
"Shh shh, it's okay Mabel." Dipper said while hugging Mabel. "I promise I'll join you within the hour. Say, why not use this opportunity to meet new some new people?"
sniff "I don't know, talking to new people is intimidating. I'd much rather spend tonight with you." lamented Mabel.
"And you will Mabel, I'll join you in a bit, and then we can have a terrific night. Actually why don't you wait here and leave when I'm ready?" asked Dipper.
"Nah, I've been cooped up inside this shack all day. I'd like to get fresh air anyway." answered Mabel.

Mabel was just out the door ready to leave.

"Well, see you in a bit bro." said Mabel.

"Yeah... I'll see you then too." replied Dipper.
"Mystery Twins?" asked Mabel.
"Mystery Twins." assured Dipper.
And with that Mabel walked out of the Mystery Shack, down the side of a street with no sidewalk, and lots of trees surrounding. The sun was just setting.

"Sigh, I don't really know why I'm going if none of my friends want to come along too." Mabel said tearfully. "It's like they don't want to hang out with me, as if they all have something better to do than hang out with Mabel or something. All I wanted was for us all to have a good time, but nope, some people have important lives with important things to do. And then there's poor old me..." Mabel slumped pretty hard, then she tripped face first into the ground.
"Ow! I think I cracked my braces." She said in pain. "What did I trip over anyway?" She turned her head and saw she tripped over a tree root.
"Huh, somehow I didn't see that big root sticking out." Mabel said to herself. "Now if I can just-"
Her foot was entangled and she couldn't get it out. Then suddenly, the root put a firmer grasp on Mabel's ankle. "Ahh! Hey what the..." said Mabel.
Then the root retracted and dragged her deep into the woods. "AAAAHH-" screamed Mabel, and then silence.
"Oof, ow where am I?" Mabel was in some kind of hole in the dirt, it was pitch dark.
"AAAHH, let go! What are you doing!? AAAAAAHHHH!" Mabel screamed, and then once again things went quiet.

Some time later, Mabel walked out of the brush and back onto the street where she fell. "Mmm, now this is more like it." said Mabel. "Let's have a little fun tonight." She cracked her neck after she finished speaking, not a common thing for Mabel to normally do. She then proceeded down to the fair as if nothing had happened. She seemed more cheery than she was before she had fallen. She had a big smile on her face, but something about her smile seemed off. Very very off. Like 'there's something fishy going on' type of off.

To be continued in Chapter 2...