Author's Note: I came up with the idea for this one-shot after seeing the teaser for the next episode of Gravity Falls; Dipper and Mabel Vs. the Future. My feels have been going crazy the last few days since I first saw it. Best I can tell, this episode will spell the end of Dipper and Mabel's friendship, as was foreshadowed at the end of A Tale of Two Stans. But what might be the consequences of this fallout? Considering how it ended between Stanley and Stanford, this might be one possibility.

Read and review... and hold on to ya' feelz.

Dipper and Mabel's 13th birthday had finally come… but it was not the day of celebration Mabel had planned.

She'd invited all their best friends in Gravity Falls; Soos and Wendy, Grenda and Candy. She knitted a new sweater for the occasion, and procured a cake from the local market… following a disastrous attempt by herself and Grunkle Stan that resulted in a kitchen fire.

Mabel planned the perfect party for herself and her twin brother; except for one problem.

Her brother didn't show up.

At least, not at first. It wasn't until many hours later, just as the sun began to set, that Dipper returned with Grunkle Ford. Neither was willing to tell where they'd been or what they'd been doing. It shouldn't have mattered; better late than never, right? But Dipper said he was tired, and that he wanted to go to bed after whatever harrowing adventure he'd returned from. He wouldn't even sit down for a minute to blow out the candles on their birthday cake.

That made Mabel angry. She was already frustrated with Dipper for not helping her with the party; he said he was too busy on his "special mission" with their great uncle Stanford, but now he didn't even want to celebrate their own 13th birthday.

It started with harsh words. Mabel accused Dipper of being selfish, and Dipper retorted by saying Mabel was being childish. The work he was doing with Grunkle Ford was important, he said. His family, his friends were also important, Mabel argued.

Things got really bad when Mabel showed him the Rift.

Dipper never told his sister about the inter-dimensional rift. She'd gone through one the journals when he failed to show up for their birthday party… then stolen it from Stanford's lab. Confronting him with it, she wanted to know why he kept it from her, why he didn't trust her.

Dipper would never answer any of her questions. He just told her to give the Rift back. "It's dangerous," he said.

Mabel refused; she wanted answers first.

In the end, Dipper tried to take it from her. But Mabel refused to let go, and so they wound up rolling around on the floor, punching and kicking one another while each kept one hand clasped onto the pressurized globe between them that contained the rift. Their friends and family surrounded them, trying to get them to break it up.

"Mabel! Give. Me. The rift!" Dipper insisted.

"You answer me first!" Mabel cried. "You can't even trust your own sister anymore!?"

"Do you have any idea how dangerous this thing is!?" Dipper retorted, trying to tug the rift out of his sister's hands. "I promised Grunkle Ford I'd keep this a secret for a reason!"

"What reason!? You tell me why you can't trust me!" Mabel demanded, tugging the Rift back towards her.

"Just. Give me. The-"

Dipper's demand was cut short as Mabel kicked him in the stomach- hard. He stumbled back, the wind knocked out of him. Mabel fell over backward, clutching the Rift to her chest, breathing heavily.

Their fight had done a number on Mabel's party decorations. The colorful paper chains were torn from the ceiling, the table holding the snacks and drinks as well their birthday cake was knocked over; the cake itself was lying on the floor, the side bearing Dipper's name in blue icing flattened to a pulp.

All of her hard work, wasted. All Mabel wanted to do was celebrate their monumental 13th year as brother and sister, but her twin had more important things to do… or maybe he just though he was too mature for a birthday party? Always running off in search of danger, pretending to be older than he really was, crushing on a girl that was nearly three years his senior...

Dipper was born five minutes after her, but Mabel was sure he liked to think that he was her "older brother"; no longer seeing her as his twin, or his equal… much less his friend.

These troubling thoughts had run rampant through Mabel's mind ever since Stanford came out of the portal, and she heard the sad tale of how he and his twin brother- their own Grunkle Stan, had been torn apart. They were once the best of friends, but they'd allowed a petty grudge to destroy that friendship, culminating in a brawl that would separate them across dimensions for 30 years. Mabel recalled asking Dipper to promise her that he'd never turn out like that.

He never really promised her that he wouldn't. In fact, he'd practically mocked her feelings, only feeding her uneasiness at the thought of losing her only brother.

Now all those feelings were boiling over, burning away all Mabel's rationality and ability to reason, leaving nothing but unbridled anger and agonizing heartbreak… born of the thought that Dipper no longer cared about her.

Mabel looked down at the Rift in her hands, and then into her brother's eyes. She saw no hint of guilt; only more anger.

"Some brother you turned out to be," Mabel spat.

At these words, Stanley's eyes widened. He was getting a serious sense of déjà vu, for he'd spoken those very same words before, to Stanford… and spent the next thirty years living with the consequences.

He knew what was coming next. In his gut, he knew, even though he barely understood what that thing in his niece's hands was.

"You care more about your dumb mysteries than your own sister…" Mabel accused. "…then you can have 'em!"

She held the Rift high above her head, and before anyone could stop her, threw it at her brother.

Dipper ducked; the rift hit the wall behind him, and burst open. At first, nothing happened… and then, a portal, shaped like an upside-down triangle and brimming with a fiery aura, erupted from the wall where the capsule had smashed.

Dipper found himself being lifted into the air, pulled towards the triangular portal by an unknown force. His arms and legs flailed pointlessly in his attempt to fight the pull of gravity.

"Dipper!"

Wendy surged forward, grabbing hold of Dipper's hands to try and pull him back… only to find herself lifted into the air as well, pulled towards the portal along with her friend. Stanford grabbed hold of her boot and tried to pull her back.

Mabel and the others could only watch in horror and shock. Stanley grabbed hold of his brother from behind to try and help him pull the two teenagers back from the portal… but Wendy's foot slipped free of her boot, and she and Dipper tumbled into the abyss.

"MABEL!"

"DIPPER, WENDY; NO!" Mabel cried. A blinding flash of light followed as her uncles stumbled back from the portal.

When Mabel's vision cleared, the portal was gone. A triangle-shaped hole has been burned through the wall where it'd been, allowing the last rays of sunlight to pour into the living room. Dipper and Wendy were nowhere to be seen.

"Dipper… Wendy? Dudes, what the heck just happened?" Soos asked, completely dumbfounded.

"The rift; it collapsed in on itself!" Stanford observed. He sounded relieved, as though he expected something much worse.

Mabel got up from her position on the living room floor, running through the hole in the wall to the outside. Dipper… and Wendy, they couldn't be gone! They just fell through the hole in the wall, that's all. Perhaps they were lying in a crumpled heap in the grass outside the Shack, but they couldn't be gone!

Mabel didn't want to accept it, couldn't accept the terrible reality of what she'd done.

But she couldn't deny it for long. For when she went outside, there was no sign of her brother or their friend. They were gone; sucked through the portal, probably into Bill's realm. The rift, unstable and thus unable to sustain itself, had collapsed behind them. There would be no way for them to come back; Mabel knew this, for she'd seen this dismantled portal in Ford's lab when she stole the rift.

Mabel fell to her knees, sitting in the grass and staring into the sunset. The anger had faded away, replaced by confusion and shock.

"Dipper… Wendy?" Mabel called out feebly.

"Dipper… no; I didn't mean it, please… no…" she stammered. But Dipper couldn't hear her. Time and space had torn them apart, and now he was lost in some distant realm, along with one of their best friends. But she didn't want to believe it. This had happened once before, and Mabel knew how that ended. But it couldn't happen to her and Dipper, not them…

Her great uncle Stanford came up beside her, and spoke the horrible truth.

"They're gone," he said simply, a hint of sorrow in his voice. "I'm sorry, Mabel…"

Mabel couldn't take it. The tears welled up inside her, and she choked back a sob; tears soon spilled down her cheeks as the reality of it dawned on her… and her heart broke for the second time that day.

"No…" she choked out, closing her eyes.

"NOOOOO!"

Author's Note: Well, that's it. I just had to get this idea out of my head. Not sure what will happen in the upcoming episode, but I have a strong feeling that Dipper and Mabel won't be friends anymore when it's over. Never mind this fan-fiction; I actually think the most likely possibility is that Dipper will stay in Gravity Falls once the summer's over, and Mabel will probably go back home. I got a hunch that the show is going to fast-forward a few years into the future after that, and we'll see Dipper and Mabel when they're 16 or so. After that... not sure what might happen. As for Bill and Lil' Gideon... no idea what might happen with them. Some people think Bill's going to be in the next episode, but I'm sure the focus will be primarily on the Mystery Twins.