Since getting into Gravity Falls and Star vs the Force of Evil, this is the story I've wanted to write. Since we'll never get an official crossover due to that odd law that Disney original shows cannot crossover, I'm going to try and make this as close to the shows as possible. Timeline wise this takes place before Bon Bon the Birthday Clown, but after all of Gravity Falls.
No matter how excited we were to spend another summer in Gravity Falls, things just weren't the same for Mabel and me. With Grunkle Stan and Grunkle Ford off sailing around the world, and Bill defeated, the town's mysteries just seemed less… mysterious.
Dipper laid on the couch of the Mystery Shack's living room, flipping through the latest issue of his favorite paranormal magazine Conspiracy Times. He had tried submitting articles about Gravity Falls in the past, but they had never accepted them. Mabel meanwhile laid on the rug nearby. She had two glue guns and was using them to frankstein-together some wooden planks and cardboard into a sort of new "pig house" for Waddles.
"Now Waddles can have his own room," Mabel looked over at her brother. "He's getting to that 'I need my privacy' phase." Waddles snorted in response.
A ring came from bell held above the door to the gift shop and customer entrance to the Mystery Shack. It had been pretty slow lately, and this was their first customer of the day.
"Can you dudes get that?" Soos called out from the kitchen. "I'm trying to stitch the Fiji mermaid back together."
Dipper put down his magazine and hopped off the couch, followed by Mabel. They walked into the gift shop to greet the customers and see if they wanted a tour.
"Hi, do you know Stanford Pines?" That certainly got the twins' attention.
Standing in front of the door were a boy and a girl roughly a year older than Dipper and Mabel. They looked like average teenagers at first, until Dipper noticed the girl had heart marks on her cheeks and was carrying some kind of toy wand. It had a broken star in the center and wings unfolding from it, she seemed a little old to be playing with a baby toy.
"Uh, he's our great-uncle," Dipper answered. He looked over to Mabel, who seemed to been enraptured by the girl. "Who are you?"
"I'm Marco," the boy answered, "and this is-"
"Star Butterfly. I'm a magical princess," the girl took her wand-toy and waved it over her head, "from another dimension!" To prove her point, the wand created several pink, fluffy creatures that scampered away while a rainbow formed above her head.
While Dipper tried to wrap his head around what has just happened, Mabel's jaw dropped and her eyes widened to the size of dinner plates. "THIS IS THE BEST DAY EVER!"
"Now give us the journals," Star held out her open hand.
"What? Who are you?" Dipper narrowed his eyes at Star and Marco. Something was very off about the two. While the idea of a magical princess from another dimension was not half as crazy as some of the things Dipper had seen last summer, one randomly showing up and demanding Grunkle Ford's journals was very, very suspicious.
"I just said. I'm Star Butterfly, a magical princess-"She started to wave her wand.
"No, no we don't need to see that again. I mean what would a magical princess want with our uncle's journals? Where did you even come from?"
"The far away land of-"
"Echo Creek, California," Marco cut off Star.
"Mabel, can I speak with you in private?" Dipper grabbed Mabel by the shoulder and led her way.
The twins stepped into the living room, just out of earshot from Star and Marco.
"What's the matter? We've got a magical princess out there!"
"A magical princess who comes out of nowhere demanding to have the journal? Doesn't that seem a little suspicious to you? Remember, 'in Gravity Falls there is no one you can trust'."
Mabel shook her head, "And in Gravity Falls, we solved all our problems by trusting other people. Let's hear her out."
"Fine," Dipper said. The walked back out to the gift shop.
"What exactly do you want with the journals?" Dipper crossed his arms and frowned. After Gideon and Bill, he'd noticed that anyone who deliberately sought out the journals tended to have less than noble intentions.
"Well that's a long story, but I'm gonna tell it anyway!" Star used her wand to poof a barstool into the room which she then sat on. "It all began long, long ago, yesterday afternoon…"
"I had just gotten home from school when I saw something pretty bad. My parents were there. Normally they only come to Earth when Dad gets on Mom's nerves or when I'm in huge trouble. Since both of them were there, I was guessing it was the latter.
Mom was all like "Blah, blah, blah, Star you're old enough now that it's time for you handle some important task blah, blah, blah."
And I then I said "I am plenty responsible, who do you think takes care of all the laser puppies?"
So Marco's mom said, "Marco does."
And I said, "That's beside the point."
And then Marco said, "Star, what I am doing in your flashback? I wasn't even there, I was at karate practice."
So I said, "Because I didn't want you to feel left out."
Anyway, my Dad said, "This is serious Star, there is a human named Stanford Pines, and old friend of ours, who has written several journals collecting information on all sorts of arcane lore."
"Like my spell book?"
"Sort of," Mom answered and she continued, "Blah, blah, we need to know something inside the journals but we can't reach Stanford, blah, blah he said if we ever needed to find him look for a place on Earth called Gravity Falls, Oregon. You're old enough now that your father and I are trusting you to handle a royal task on your own."
Then Marco's dad said, "But Star is still a child, princess or not, she can't just go to another state all by herself!"
Then Marco came home and was in the flashback for real and he was so confused so my dad said, "Marco can go with you then. He can be your squire!" And Marco was still really confused.
So then we got packed and went on a bus, then we got tired and slept. Then I woke up at about three AM, I was really hungry but the bus driver said that the next rest stop was an hour away. So I-"
"Star," Marco interrupted, "You don't have to tell them every single thing we did on the way here."
"Oooh," Star nodded, "Gotcha. So anyway after we put the fires out…"
"We get the idea," Dipper said. "Look, I'm sorry but we can't help you. Grunkle Ford isn't here and the journals are gone."
Star's near permanent smile faded, and Dipper swore he saw her heart marks crack slightly. "Well, thanks anyway. Come on Marco, let's get back on the bus. Hopefully my mom won't be too mad."
Marco held the door open for Star but before they could leave Mabel shouted, "Wait! We can call Grunkle Ford and you can stay with us in the meantime."
"Mabel!" Dipper scolded.
Star's smile came back and she hi-fived Mabel. "Thanks! I promise you won't regret this!"
Marco looked between Star and Mabel and Dipper. Dipper glared at him in return. "I'll just… go get our stuff from the bus then."
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