Heya! Before we start I just want to say: this is my first crossover and my first story without an OC. I'll be honest: I'm complete Undertale Trash and have been for quite a few months now... and now I'm slowly sinking deeper into the unique (and bottomless) pit that is the fandom of Gravity Falls! Yeah, it was about a week and a half ago that I watched and finished GF for the first time and... yes, I cried at the last two episodes, YOU GOT A PROBLEM WITH THAT?!... ahem... in any case! I was just sitting here, thinking of ideas on something to write involving one thing or the other when suddenly I thought: "Hey, why not a story with both?" ... and thus, here it is! xD

I've only seen two or three crossovers involving a post-pacifist story, in which the monsters of the Underground go to the surface and find themselves in the curious little town of Gravity Falls. So I thought it would be great to write one of my own! This prologue is just to give you a little insight on the situation Underground. A bit of backstory on one side of this story and an ending that will lead to the main plot.

Also, I'm referring to Frisk as 'they' for now, to keep up the ambiguity of their gender until one of the next chapters (don't ask why, just go with it).

Please enjoy~!

"Normal dialog"
'A character's thoughts'

Prologue

They had done it. They had finally done it. Frisk won, and with Asriel's help, the barrier was broken.

Everyone in the Underground was happy! Asgore Dreemurr, that fuzzy pushover that was king of all monsters, called every monster to celebrate the little human's victory. For the sake of the celebration, Toriel tried her best to tolerate being near Asgore; Undyne and Alphys were spending quality time together; Mettaton (with his now fully-functional body) dragged Napstablook along, who for the first time in a long while seemed so happy; Papyrus offered to cook some of his special spaghetti… of course, for everyone's wellbeing, Sans decided to distract him to take his mind off of his cooking plan.

And speaking of the comic, Sans was smiling a genuine grin this time; so far, the human hasn't restarted the timeline yet. That alone seemed like a good sign. Just a timeline ago, him and Frisk had a problem with a corrupted entity hell-bent on destroying everything and everyone. After hours and hours of research, Sans found a solution and got Frisk (the small part that was left of them, at least) to cooperate. Eventually, Chara left Frisk's body and disappeared off to God-knows-where.

Right now, Sans was sitting off to the side, smiling as he watched his brother interact with the others (and enjoying the attention the little Monster Kid was giving him). As he looked around, he noticed the center of the party was missing. From the corner of his eye-sockets, he saw a blue and magenta blur passing through the door. His sockets narrowed in confusion and, once he was sure no one was watching, teleported himself at the end of the Hall.

Ah, the Judgment Hall. This place brought a lot of memories for Frisk; some were good… the other 358 times, however, were the most frustrating and heart-wrenching moments they've ever lived. And that's saying something from the one who decided to run away from the orphanage that raised them.

Indeed, little Frisk was an orphan since they were 6 years old. The blurry memories they have before arriving to the orphanage are those of a loving mother and father who loved their sweet and silent child with all of their hearts. Everything was going well for the small family, until their car got crashed on their way to the small town of Ebott at the base of the mountain. Both mother and father didn't make it, and Frisk secluded themselves from the rest of the children.

Ever since then, most of the kids of the orphanage either ignore Frisk's existence or bully the silent child. One day, Frisk decided enough was enough and ran off to the mysterious Mount Ebott. Regardless of what happened to them, Frisk knew it would be better than living in that hellish place.

And now, here they were: countless of resets later, leaving the celebration of a pacific victory... walking through the one place that still brings them a certain sense of dread. Now, Frisk was happy, and everyone else was happy… but they felt like they didn't deserve it. There was only one person left to save… maybe there was no way to save him, but there's nothing to lose with just trying, right?

Frisk was already halfway through the hall, when the air shifted in front of them.

"heya, kid" Sans was smiling as usual, but his eye-sockets held a light of concern. "i noticed you were leaving, and i decided to come along with 'ya. y'know, so you don't feel bonely" Frisk giggled and that sound helped to calm Sans' non-existent nerves.

"I'm okay, Sans" assured the quiet voice of his small friend.

"good to know, kiddo" Sans looked around the room, sockets slowing when he glanced at the windows letting the twilight shine through. "y'know, it's a bit humerus, but what little i remember of us standing here, we're in swapped places" Sans seemed to breath in and out slowly (how does he do that?! 'Magic', he says), closing his sockets, soaking in the calm and quiet. "… but this time… i feel at peace. of course, there's still a chance for you to reset everything, and we'll do this all over again, but still…" he was cut off at the sound of small steps getting closer.

"Sans, I told you before…" Sans tensed up at the feeling of thin arms around him, before he returned the hug tightly. "This one's the last. I promise you, this is the one. When the party's over, and after we have slept some hours, we're all gonna watch the sunrise! Just wait 'till you see it, it's beautiful!" the kid explained, moving their arms to emphasize their excitement.

Sans chuckled. Seeing Frisk's seriousness melting away to show their happier, more childlike side… it just made him smile no matter the circumstances.

"heh, just be sure to wake me up just in time to see it, ok?"

Frisk simply gave him a thumbs up before hugging him again and walking further away from the party.

"so, where 'ya goin', buddy?"

"…" Frisk thought carefully their next words "It's just… there's someone else I want to help"

"oh? mind if i tag along, kid? tibia-nest i just wanna keep you company"

"Well…" the little brunette trailed off when an idea came. "Oh! Can you take us to the Ruins? I just want to get something from mom's house and then go talk to someone at the end of the cave"

"ok. hold on tight, kiddo; we're taking a shortcut" Frisk closed their eyes and Sans immediately got them inside the Ruins, in front of a tree just hiding Toriel's house from view. "so here's where tori lives? seems pretty cozy"

Frisk nodded happily. "I'll be right back!" they sprinted to the small house and searched through the few things left behind for the item she was searching.

Meanwhile, Sans was waiting outside, leaning on the bark of the tree while lazily twirling a small bone in his hand. He looked up when he heard the child's footsteps.

"that was fast…" he trailed off when he saw the item. "… uh, kid, why do you need an empty flower pot?" Frisk just smiled and tugged at Sans' sleeves to a certain path. "ok ok, i'm coming frisk" he chuckled and followed the energetic human.

Walking the long way, they arrived to the end of the cave. The rocky walls were humid; the floor was filled with grass, vines, and golden flowers growing all over the place. The most eye-catching spot was the center of the clearing, where a perfectly shaped flower bed was illuminated by the moonlight shining through the big hole at the ceiling of the cave (which served to tell Sans all of the time they took… 'Paps must be really worried right now…'). That wasn't the only thing; over the rest of the flowers, stood a taller one with a familiar face watching them approaching.

"Oh, what a surprise! But if it isn't the idiot and the smiling trashbag!" Flowey sneered at them.

"heh, nice to see you're doing well, weed. why do 'ya have that look on your face? did your parents ground you?"

Flowey cringed. "Ugh, don't start with the bad jokes! Hey Frisk, just be quick and tell me what the heck do you want?"

Frisk smiled and showed him the pot they brought. "I'm not leaving you behind. You know that, right?"

Flowey looked genuinely shocked. "What?! But… after everything I did?... Even after I already told you there's nothing you can do for me?!" Flowey looked angry and… desperate. He knew there was no way for him to be his old self again, and he didn't want his parents to know, but…

Sans was startled at the sight of the flower crying.

"I-I don't want to be alone anymore... why?... how am I feeling anything right now?!" he was confused, angry, sad, and a lot more at the same time.

"I'll take you with me! You won't be alone anymore… Asriel, I promise…" Flowey looked up with teary and hopeful eyes.

For Sans, the next few minutes were a blur. Frisk carefully put Flowey (or Asriel. 'God, that thing is the prince?'). Then Sans took the two (now three) of them back to just outside the room of the party. The place had quieted down and some monsters were arranging some sleeping bags and pillows for the monsters to sleep in.

And just as Sans thought, Papyrus ran up to them, asking them where they had been and happily greeting the flower (action that made Sans' eye-socket twitch and flash briefly).

Once everyone in the huge room settled in, the lights went off and everyone drifted off to sleep; all with the excitement of what tomorrow would bring.

Next chapter:

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