Cursing warning on this one, for the children.

Nobody said anything after Sans finished explaining the current situation.

"So basically," he said, "I think this whole mess with time, determination and Chara has left Frisk with some serious issues, not the least of which is their severe 'developmental temporal displacement.'"

"YOU JUST MADE THAT UP, DIDN'T YOU SANS?" Papyrus challenged. Sans just winked.

"The term, yes. The phenomenon, no. The gist of the problem is that Frisk spent, by my calculation, roughly five years stuck in that time loop, leaving their mind to develop without letting their body catch up."

"W-w-well wait a m-minute," Alphys stammered. "If that's the case, then shouldn't y-you and Asriel have been affected, too?" Sans nodded, considering the question.

"Good point. Well, the difference between the two of us and Frisk is that our temporal displacement constitutes a much smaller fraction of how long we've been alive than it did for Frisk. I'm displaced by over a decade, but that's only like, four percent of my current age. It would've taken centuries to really mess me up. Frisk, on the other hand, was displaced by over fifty percent of their age, which has got to cause some serious psychological problems. They had to mature emotionally without their brain being able to physically develop." Silence followed.

"YOU KNOW, SANS," Papyrus said, trying to lighten the mood, "YOU'RE SO LAZY THAT I SOMETIMES FORGET YOU'RE A SCIENTIST." Sans' grin broadened slightly, but nobody else seemed too amused.

"And that's only one part of the problem," Asgore interjected. "You and Asriel weren't possessed by a demon for any length of time, let alone years."

Toriel had been quietly sobbing over the pain Frisk must have endured, but when she heard Asgore use the word 'demon,' her head snapped up, rage in her teary eyes.

"Demon?" she hissed. "Asgore, that's Chara you're talking about! Our child! How could you say that?" She buried her face in her hands, wanting to just block everything out.

"Actually," Sans said gently, "Asgore might not be too far off the mark." When Toriel shot him an accusatory glare, Sans held up his hands in a defensive gesture. "I'm sorry, Toriel, but it's true. When a soul exists outside a body, it starts to break down. That's why the souls of the six children had to be kept in those special tanks. They would have disappeared over time if we just left them. From what I've gathered, Chara's soul has been persisting on its own for a long, long time. Whatever that... thing, is, it's not Chara anymore, not really."

Toriel didn't challenge Sans, but her tears made it quite clear that she wasn't ready to accept his words as the truth just yet.

"So the problem," Frisk interrupted sadly, "Is that you all think I'm broken and you have no idea how to put this humpty dumpty back together again. I didn't ask for any of this, but here I am. I'm afraid of myself now. I don't even want to think about what I would have done to Reggie if Asriel hadn't interrupted."

"Speaking of," Asgore asked, "Why did you stop when you saw Asriel, anyway?"

Frisk scrunched up their face, thinking.

"I don't know," they said. "I just saw him and sort of… froze, I guess. Not sure why."

Again, silence.

"Frisk," Sans slowly said, "Would you mind if I took a look at your soul? Something about this situation seems a bit off to me."

Frisk thought for a moment, then nodded, a concerned look on their face.

Sans extended a skeletal hand, and with a ping, Frisk's soul materialized, red and pulsating. Everything looked normal, except…

"Frisk, What. Is. That?"

Around the edges of Frisk soul sat a glob of what looked like black sludge.

A brief look of horror crossed Frisk's face, but then they froze and their expression went entirely blank. In an instant, the sludge encompassed Frisk's soul, turning it pitch black and, their soul rocketed back inside their body. They shook violently for a few moments, then went still. Slowly, an insane grin spread across their face.

"Took you long enough to notice, Sans," they laughed. "Though I am a bit disappointed. I was hoping to see how you'd handle seeing your precious, innocent Frisk murdering one of their classmates, but then you had to go and find me out."

"Chara," Sans sneered. "Why are you here? What do you want?"

"Ha! Nice act, tough guy! For a second there, you were almost intimidating, but your shaking knees gave it away." Sans' knees were indeed wobbling. "As for why I'm here, I was just hitching a ride on Frisk, causing a little mayhem as I went along until you guys found me. Here's the thing. You were right. I do want an adult body. Why am I telling you? Because I know there is nothing you can do to stop me, and I just wanted to watch you squirm before I started looking for new housing. Oh, and while I'm here, here's something to remember me by."

Without warning, Frisk's hand slipped down to their waist and retrieved a hidden kitchen knife. They brandished it for a moment, then flipped it around and sent it plunging toward Frisk's gut. Sans' eye flared blue and the knife was enveloped in a blue shroud, stopping inches away from Frisk's body. Chara, the puppeteer, simply smiled even wider.

"Neat trick, pal. But I've gotten a little bit stronger since we last met."

With a flick of their wrist, the blue aura shattered and the knife resumed its deadly descent. And once again, it stopped inches away from Frisk. Now Chara frowned.

"What is it this time?" They paused again, frown deepening as their wrist began to shake. "Oho! Showing a bit of fight, are we?" Their expression shifted again, to a look of determined anger.

"Chara," Frisk hissed through gritted teeth, once again in control, "get out. We used to be compatible, but we're not anymore." To back up Frisk's words, the knife began to slowly, inexorably move away from their body. The silent battle continued for what seemed like hours, Frisk's fingers slowly peeling off the knife one at a time until it fell from their grasp. As soon as the knife hit the ground, a dark essence surged out of Frisk's chest, disappearing into the air.

"Well that was unexpected," Chara's disembodied voice chuckled. "Didn't think you had it in you to banish me. Still, you're all doomed once I find an adult body." Everyone could hear their insane laughter receding until it finally vanished. Silence seemed to be a theme on that day, because once again the room went quiet. Finally, Sans spoke.

"Well, that's concerning, isn't it? The creepy stuff aside, they broke my magic. They shouldn't even have been able to do that."

"I can only imagine what they'll do to the the others," Frisk said, still shaking from their ordeal, "if you're the strongest monster alive and they could break your magic that easily." Their words were met by massive protest from Asgore, and Undyne especially.

"Sans," Undyne snarled, "After we destroy that creepy punk you and Frisk owe me a fight. A real fight."

"Whoa, hey," Frisk said, waving their hands at Undyne. "Trust me, you do not want a real fight with Sans." Before Undyne could volley a retort, Asgore cut in.

"What I don't understand is why we're talking about who's stronger than who when Frisk was just possessed violently and almost stabbed to death. Frisk, how are you feeling?" Frisk considered the question.

"You know, even though the adrenaline is subsiding, I feel surprisingly ok. I guess that tends to happen when a demonic entity suddenly stops leeching off of your life essence. But that's not really important right now. Seriously, that display should have all of you on edge. I know none of you has seen it before, but Sans has the strongest magic I've ever seen. Asriel's wasn't that much stronger even after absorbing seven human souls, and that's just in raw power. Sans has a much higher degree of control, and it worries me very much that Chara could just break it like that."

Asgore eyed Sans with a look somewhere between admiration and suspicion. "Well that's certainly impressive. How did you even get your magic to be that strong, Sans?"

Sans' pupils disappeared, a clear signal that the topic was off-limits. "That's not too important right now, King Fluffybuns. What is important is dealing with this monster, and right now, I only have one idea how to do it." He paused for dramatic effect.

"W-well?" Alphys asked.

"We've got to enlist the monster who knows the most about determination and human souls."

"And who might that be, darling?" Mettaton chimed in, loving the theatrics. Sans paused again.

"Me and Paps's dad. W.D Gaster."

Everyone in the room except Frisk and Sans was hit with a wave of deja vu at the mention of the name.

"You and Papyrus have a dad?" Alphys asked. "That's, uhh, unexpected. Also, I feel like I've heard the name before but I can't place it."

"Not surprising," Sans muttered. "Everyone in this room was personally acquainted with Gaster. He was the Royal scientist before Alphys, and I worked with him. After the humans started falling down here, one of the more violent children tried to fight him… Heh. Anyway, as they were fighting, he noticed that the kid seemed to know what he was going to do before he even did it. He asked the kid about it, and he told him that it was really the, what, fiftieth time they had fought, and explained the resets. The kid kept trying and trying until his determination gave out and he died permanently. Now, to Gaster, that all seemed to happen in one fight, but it really took quite a few. Curiosity piqued, he started studying the kid's soul and the other soul that had been collected so far and wound up discovering determination. Then Paps n' me came into the picture. One thing led to another and he started studying time travel. Then, during an experiment, he basically wiped himself and four other monsters from reality. That's why you recognize the name but remember nothing."

"Oh, of course," Undyne sighed. "Of course there's a mystery guy who tore the fabric of reality and erased himself from existence. Anyone want to drop any more bombshells today?"

Without hesitation, Frisk piped up. "I'm actually a girl!"

"Really?" Asked Undyne.

"No."

"Then why would you say that?!"

"I thought it would be funny."

"Well it wasn't you little -"

"Quiet!" Alphys interrupted. "Sans, if Gaster were wiped from reality, then how do you remember him, and how do you plan to get him back?"

Sans gave a mirthless laugh. "That's the question, isn't it? I've been trying to figure that out for years. I might have something, but it's a stretch at best. I can show you if you want. Oh, and Asgore, you can come too. I might need your help with it."

Asgore stared at Sans for a moment, perplexed. "The sciences have never been my strong suit, but if you believe I can aid you then I will accompany you."

"Cool," sans said. "Frisk, put your hand on my back. We're gonna take a shortcut."

Frisk promptly complied as Sans put his hands on Alphys's and Asgore's backs, respectively. Then, with a ping, the four winked out of existence.

Once again, with a ping, they all reappeared at the rim of a gaping hole. Asgore looked around and realized from the view that they were standing on Mt. Ebbot.

"If this is Mt. Ebbot, then that hole must lead-"

"To the Underground, yeah," Sans finished. "Sorry, I would have made the trip in one jump if I could have, but taking three others with me makes it kinda hard."

Again, the four disappeared. This time, they appeared outside Sans and Papyrus's old house in Snowdin.

"Wait here," Sans said as he went inside.

The other three looked around. Snowdin, the town that used to be so full of life and energy, a bustling hub of the Underground, was deserted. It felt so lonely, almost haunted. After the barrier had broken, every single monster in the underground had moved to the surface leaving the underground full of empty towns and abandoned memories. Frisk shivered unconsciously. Alphys looked uncomfortable. Asgore just stared, stunned. For so many years he had ruled alone from his castle, not wanting the citizens of the underground to see how broken he had become, and now, the first time he visits a undertown in decades, he finds it an empty, abandoned husk of what is used to be. It was a rather sobering moment. Abruptly, the door Sans had passed through flew open again with a bang. Frisk noticed he was carrying a key.

"Come on," he waved for the others to follow. They went around to the back of the house where Sans ripped off some siding and revealed a hidden door. He inserted the key into the lock, turned it slowly and was rewarded with a loud click. Almost of its own bidding, the door swung inward, creaking ominously. Sans strode into the room purposefully, flicking on a lightswitch without looking as he went. As the fluorescent lightbulbs flickered on, the group were greeted with the sight of a small laboratory. It was mostly bare, decorated with a few workbenches covered with tools, but Sans ignored all that as he beelined to a large object covered with a dusty tarp that sat in the rear corner of the room. He yanked off the tarp with a flourish, revealing an odd looking machine with some sort of monitor. Without hesitation, Sans punched a button and the machine whirred to life, a single green bar blinking on the screen. Sans started furiously typing commands, talking as he went.

"So I've done a lot of research on what happened to Gaster, and I'm pretty sure I've figured out exactly what happened. I think he succeeded in going back in time, but I don't think he went back in our time. Basically, multiverse theory's a bitch. Sorry, Frisk," he added as an afterthought. "I've gone over the data so many times, and everytime I look at it that's the conclusion I draw. I think he went back in time but accidentally switched timelines and jumped to an alternate universe. I've been trying to isolate which one he jumped to, but there are so, so many." He tapped a few more keys and the screen changed again. "Alphys, if you could take a look at this data and verify my theory I'd appreciate it. If we do anything to get Gaster back, it's gonna involve jumping across universes, and I don't want to try that without a second opinion."

Alphys nodded, and Sans surrendered his seat to let her go through the raw data. He hovered over her shoulder and they started talking scientific jargon that went far over Frisk and Asgore's heads.

"So," Asgore began, "if we make it out of this alive, would you like to come over to my house for tea?"

"You're inviting me to a tea party?" Frisk snorted, grinning widely. "You, the giant ripped goat monster, is asking someone to a tea party? Heck yeah! Sounds awesome!"

Asgore smiled tiredly.

"Thank you, Frisk. You know, I thought things would be better after we got out of the underground, but now that I have to fight the demonic soul of my dead child, I'm not so sure."

Frisk's eyes dropped to the ground.

"Yeah… Things seem to have a nasty habit of taking a turn for the worst, don't they? But the best we can do is keep trying."

Asgore eyed Frisk, once again struck by how strong Frisk was. He dropped a paw onto their shoulder.

"You're right, Frisk… Thanks." After a brief pause, he turned over his shoulder and called to Sans, "by the way, Sans, why did you need my help, anyway? I can see why you'd want Alphys and Frisk, but I don't see how I fit in here." Sans turned away from the screen.

"We're probably going to be jumping into a different universe, and there are some dangerous ones. Underfell, Underfresh," Sans shuddered, "and we might need some muscle for that. Nobody can beat my magic, but you're the physically strongest monster there is, Asgore."

Asgore nodded, accepting the explanation.

"Sans," Alphys said, looking up from the screen, "I haven't gone through too much of this, but from the way it's looking, I'm positive you're correct about the alternate universe."

"Great," Sans grumbled. "I was kinda hoping you might come up with a better solution, but oh well. What can ya do. Right," he said, clapping his hands together, "That pretty much whittles our options down to one: Universe hopping. This machine can do it, I just haven't tried it before because I never had anyone who could pull me back, but Alphys, if you can operate the machine you can send the three of us over and bring us back."

"Um," Alphys muttered, "Doesn't this seem a bit rushed to you? And how am I even supposed to know when to pull you guys back? We won't exactly be able to talk to each other after I beam you over."

"I figure you bring us back after a week. If we don't have Gaster we can always go for another week."

"Um, ok. But doesn't this still seem rushed?"

"Look, Alphys, I'd love to waste time with goodbyes, but Chara is out there looking for a body and we don't have time to waste. You can make some calls if you want to explain what we're doing, but we've got to get going on this ASAP."

"Ok, ok. Fine. Just show me how to work this and I'll do my part."

Sans nodded and took a few minutes to give Aphys a crash-course in operating the interdimensional rift-maker. After crunching some numbers, Sans and Alphys concluded that Gaster had probably landed in dimension UAC-1. Sans had identified more than fifty universes over the course of his research, but the data only pointed at that specific dimension.

"Well, uh, make sure you're all touching each other in some way and let's make this thing happen?" she said uncertainly. Growing serious, she continued, "I'll bring you back in one week. When that time comes, make sure you're in the same area that you first appeared in, or the machine might not be able to get a lock on you. That being said… good luck, all of you."

The three nodded grimly and joined hands as Alphys punched some buttons. With a crackle and a flash, they were gone.

So hey, this story passed the 1,000 view marker! I don't know if that's a lot for a fanfiction, but I'm excited anyway. I think that's awesome! Thanks, you guys! Also, don't forget to drop a review!