Hello! This is just a quick one-shot, but it turned out well enough that I decided to go ahead and post it here as its own story. Just as a heads-up, this story features a pacifist!Frisk (As in, they've never done the genocide route at all) who uses they/them pronouns. I started writing it with female Frisk, but it just didn't sound right, so I changed it and went with they.

Enjoy!

-Daricio

Frisk could barely restrain themselves from skipping as they led Sans down the hallway of Alphys' lab, their hands planted firmly across his eye sockets to block his vision. The skeleton seemed amused by their antics, hands stuffed casually in his pockets as he walked forward, letting them steer him wherever they pleased. He even let them make him walk into walls and doorframes a few times, though by the way his smile widened each time they did it, they could guess that he could sense their pranks coming.

Both of them were in high spirits, having just come from a small, close-friends-and-family-only party for Frisk's 19th birthday, and now was the perfect time for them to give Sans a gift, too, even though it was their birthday and not his. It was the perfect time anyway.

"Surprise!" Frisk pulled their hands away, finally letting him see the 'mysterious' project they had been working on with Alphys for forever now. It wasn't done yet, but it was getting close, and the human just couldn't keep it a secret anymore. Excited, they stepped to one side so they could watch his expression, see if he recognized it before they told him what it was.

When they saw his eyes, however, they stopped dead, their grin fading. Though his smile remained frozen almost unnaturally on his face, his features felt blank, his eyelights completely dark. An almost imperceptible sense of doom began to spread through the room, and he remained silent for far too long.

"kid..." he finally said tightly, not looking away from the complicated machinery in front of him. "what is this all about?"

Frisk took an automatic step away from him. "I... You don't like it? What's...?"

Eyes still ominously dark, Sans pulled his left hand out of his hoodie pocket and lifted it into the air. Magic surged around him as he summoned a giant skull cannon, already charging for a blast of energy, its wide maw pointed directly at the machine they were almost done fixing.

"Wait, stop! Sans, what are you doing?" Without thinking, Frisk ran at him, trying to tackle him to the ground, push him away, anything to snap him out of whatever had just come over him. To their surprise, he moved deftly out of the way, much faster than they'd ever seen him move before, and they wound up sprawled on the floor. "Stop!"

To his credit, he listened to them, slowly letting his hand drop, and his strange, scary attack faded away. He breathed heavily, his dark gaze now pointed at the floor as both hands clenched into fists. "why? why would you put this thing back together?"

Frisk stared at him from their place on the ground, propping themselves up on their elbows. "Sans... I found the blueprints in your lab. You were trying to fix it before, right? I... I just wanted..."

Sans chuckled, but it came out hollow and cold. "...you haven't been able to SAVE or LOAD since we came to the surface, right? it only ever worked in the underground. now you want it back."

Baffled, Frisk let their face settle into a neutral expression, slowly rising to a sitting position. "Well... I mean, yeah. But... I thought you'd be happy about this. I thought you wanted-"

"don't you know why i built that machine in the first place?" Sans asked harshly. When they didn't answer right away, he took a breath and finally looked at them.

After a moment, the lights in his eyes flickered back into existence and he slumped against a nearby wall, looking pained. "guess you don't... okay, then... frisk, even before you showed up, there was something—an anomaly—messing with our timeline."

Frisk nodded. Flowey, of course, but they weren't going to say so. Instead they stayed silent, waiting for Sans to keep going.

It took him a while, but eventually, he sighed. "at first, we only knew about it from what we could see on our monitors, along with the déjà vu stuff. some of the stuff the anomaly did, or what little we could tell for sure it was doing, was enough to warrant trying to make it stop."

"Who's 'we'?" Frisk asked softly, tilting their head to one side. They had to be careful about how they asked this; Sans didn't like to talk about himself, and after that display, they didn't want him dodging questions. He'd hinted to them before that he knew something about the resets, but they had never really gotten a straight answer out of him. This was a first. "Someone was working with you? But Alphys said she'd never seen these blueprints before I showed them to her."

Sans smiled, giving them a sly wink. "yup. guess it wasn't her, then. anyway, to stop the anomaly, we figured we could try to hijack the resets ourselves. so we built this thing, trying to access that ability to SAVE." He jerked a thumb back at the machine without looking at it. "it worked... once."

And of course Sans was dodging their questions anyway. Frisk frowned at him. "Just once?"

"just once. but that was enough to screw everything up." Sans sighed, grin still planted on his face as his eyelights darkened again. "the point is, kid, we weren't trying to hijack the resets so that we could use the power. we wanted to block the anomaly from using it. we were trying to stop the resets. and when you came and freed monsterkind from the underground, you finally succeeded where we'd failed. the resets stopped."

Frisk went very still. "I... I'm sorry. I didn't realize. But Sans, I need this ability."

"no. you really don't."

Vehemently, Frisk shook their head. "You don't understand. If I can't reset-"

Sans was suddenly standing right beside them, one hand gripping their shoulder like a vice. "if you can't reset, then you can't treat everyone like puppets. like toys."

Frisk gaped at him. They'd never seen Sans like this before. "I... I would never..."

"but you did. reset after reset, over and over. you can't just keep treating life like it's some... some game that you have to get the perfect ending for!" Sans' grip tightened. "that's not how life works! that's not how life is supposed to work."

"If I hadn't been able to reset, Toriel would be dead right now!" Frisk burst out, tears coming to their eyes.

That stopped Sans in his tracks for a moment. He stared blankly down at them, and they went on, unable to meet his gaze. "W-when she first found me, she tried to keep me from leaving the Ruins, but I was determined to get out of the Underground. She told me to prove myself, and she attacked me. I... I didn't know what to do, I was just a kid. I figured if I showed her I could fight, she'd let me pass. But..." Frisk bit their lip, their voice dropping to a whisper. "I didn't realize how easy it is for a human to kill a monster... I didn't mean to do it. I felt... like the scum of the earth."

Sans didn't respond. Frisk couldn't even begin to guess what was going through his mind. They couldn't stop thinking about the judgement Sans had passed on them near the end of their journey: "you never gained any EXP." Only technically true. Their sins crawled on their back.

"I was... pretty horrified, but I pressed on. I met you and Papyrus, but somewhere along the line, a Chilldrake caught me off guard, and I died." The fact that they'd died multiple times before was something that would always be weird to tell people. "And a moment later, I found myself back at my save point, before I fought Toriel. This time, I got past her without hurting her. So, you see? We wouldn't be here like this if I couldn't reset."

With a heavy sigh, Sans finally released their shoulder, turning away. "... you're right, we wouldn't. but you're still wrong."

Frisk could only stare at him. "So... I should have just left it the way it went the first time?" they asked incredulously, a little hurt. "Toriel dead, and me dead?"

"look, i'm not saying that all this," he gestured vaguely around him, "isn't the best 'ending' to the underground arc of our lives. because it is. good job, kid. really."

"Then how about what I want the reset powers for now?" Frisk demanded. "Not sure if you've noticed, but being the Ambassador for Monsters isn't easy. Every day, I screw something up, and one of these times, someone's going to get hurt and I won't be able to fix it!"

Sans slumped a little, both hands sliding automatically into his pockets. "yeah. and that's how life's supposed to work. actions should have consequences, and nobody should have the power to simply avoid them. to sweep them under the rug like they never happened. that kind of power... is the reason why i can't even count the number of times i've watched papyrus die right before my eyes."

Automatically, Frisk opened their mouth, but quickly found that there were no words to respond to that with. Their mouth closed again. Opened for another try. Closed. They knew Flowey had done some terrible things, but...

Sans let his words sink in for a minute, then turned to glance at Frisk's expression. "heh. yeah, i know that you never hurt my brother. but you could have. you could have done anything and gotten away with it. you can't tell me you've never wondered what would happen if you, say, punched jerry in the face, then reloaded so he wouldn't remember. or tried ignoring greater dog when it really wanted you to play with it, just so you could watch it get desperate. hell, i already know you experimented with the different options of that 'date' with alphys, among other things, didn't you? doesn't hurt them any, right?"

He paused to catch his breath, eyes falling shut, before continuing, "you see, kiddo... if there are no consequences... nothing matters anymore. nobody matters anymore."

"I... That's not true," Frisk managed to croak out, throat dry. "I care about my friends."

With a sigh, Sans finally faced them again, his eyes brightening a little as his usual, relaxed smile returned to his face. "i know you do," he admitted. "you're a good kid. but frisk, you have to understand something: everyone makes mistakes. and that's okay. mistakes are the only way we learn things."

Frisk slowly shook their head. "I don't want anyone else to suffer because of my mistakes."

Carefully, Sans seated himself on the floor beside them, looking up at the machine with a far-off expression. "sometimes you can't help that. but knowing that your mistakes could hurt others, and that you can't take anything back... well, isn't that just motivation to improve yourself? to be a better person? if your mistakes never affect anyone else, what reason do you have to try to be good all the time?"

It took Frisk a long time to come up with a response to that, and the room fell silent as they both stared wordlessly at the machine.

Finally, Frisk ventured, "Even if nobody else remembers... I remember. That's reason enough for me to be as good as I can."

Sans glanced up with a smirk, then shook his head, chuckling to himself. Then, he reached over and clapped them on the back. "heh. i guess we can all count ourselves lucky that you think so."

Pulling himself to his feet, the skeleton cast one last look at Frisk's project before walking slowly to the door. "welp. in the end, i can't actually make you do anything, especially not once you've got your mind set. i've given my two cents, and as long as you consider what i said, i'm not going to keep you from finishing this thing if that's what you really want."

His unspoken implication that he'd rather see the machine destroyed hung in the air between them, and Frisk let out a sigh. They'd always liked Sans, and seeing such obvious disapproval from him hurt. But they couldn't just give up on this, and both of them knew it.

"don't worry, kid," Sans said with a wink. "if you were to ever take things too far the wrong direction, i'd stop you. promise. just don't make me regret that."

Before Frisk could say anything more, he stepped out of the room and was gone.