After lunch, Frisk badly needed to visit the toilet. They were briefly too embarrassed to do so, painfully aware of the three monsters watching them at all times, but eventually they figured they'd have to do this eventually, so there was no point in stalling. Papyrus went on a long string of questions regarding human bathroom etiquette at first, but sensing Frisk's strong unease (along with a curt "Shut up" from Undyne) he decided to give it a rest. Frisk did their business in silence after that. Only once they were fully clothed and outside did anyone have the nerve to speak up.

"Eugh. This is what digestion is like? That's disgusting," Undyne complained.

"SO THIS IS WHAT EXCUSING YOURSELF TO THE BATHROOM REALLY MEANS?! IF I'D KNOWN THAT, I'D HAVE CHOSEN MY WORDS MORE CAREFULLY!"

Look, it's something I have to do, okay? I can't help it either. And get used to it, because this happens several times a day.

"Yech. I hope we can figure this out soon, I don't think I can deal with that for long."

They spent the rest of the afternoon idling about in the living room. Undyne had asked to practice walking again a couple times; each time went a little smoother, but according to her it was still very slow and tiring. "It's like wearing a heavy suit of armor at all times. Good thing that's how I normally practice anyway! Fuhuhuhu!"

By the evening, while they were eating microwaved leftovers, Alphys finally popped up. After explaining the situation to her, she immediately got to brainstorming ("I-I'll see what I can figure out, but no promises."). Frisk's mind filled up with thoughts and concepts they couldn't hope to understand about magic and physics and reality and mathematics and all sorts of other sciences they didn't even recognize. It briefly disoriented them until they settled for ignoring the foreign thoughts as background noise and focusing on their plate instead.

Eventually Alphys gave it a rest for the moment, which was fine with Frisk; they could use some time out. They spent the evening lazily channel surfing on TV. Finally, late in the evening, the front door opened and their mom wearily stepped into the apartment. "Hello dear, how was today?"

"GREETINGS, FRISK'S MOTHER! NICE TO MEET YOU!"

Frisk briefly panicked, thinking their mom could hear Papyrus. Only several seconds later did they realize that, no, of course she couldn't.

"Frisk? Hello?"

Frisk shook off their thoughts and finally addressed their mother. "Oh, uh, hi mom."

Mom smiled at them, a curious look in her eyes. "What's on your mind?"

Oh, you'd never believe that even if I told you. Sans chuckled a bit at that quip. "Nothing, mom, just tired I guess."

"If you say so, honey." She looked at the TV, then back at Frisk. "Don't tell me you've only been doing this all day."

Frisk shrugged; they'd like to say no, but that would involve explaining what they had been doing.

Mom sighed. "I spend all week working to keep your lazy butt healthy and fed while you waste your time hanging out in front of that thing? You're a smart kid, you know what comes of that, right?" Frisk felt a little bit guilty for lying to their mom, but said nothing.

"HEY FRISK, CAN I TALK TO YOUR MOM? I'D LIKE TO SAY HI!"

Papyrus, no, we can't let her know you guys are here.

"WHAT? WHY NOT?"

Because I don't think she'd be okay with having you around. If she'd even believe you exist.

"OF COURSE I EXIST! HOW COULD ANYBODY DENY THAT?! FOR THAT MATTER, WHO WOULDN'T BE OKAY WITH HAVING SUCH A COOL AND FRIENDLY SKELETON AROUND?"

"bro, give it a rest. i don't think that's a good idea either."

"BUT WHY NOT?"

"Frisk? Are you all right?" Mom's voice brought Frisk back to the outside world with a start.

"Huh? What?"

Mom gave them a worried look. "That's the second time you spaced out on me already. I haven't even been in for five minutes." She looked at the clock. "Are you sure you're just tired? You're not getting sick, are you?"

Frisk groggily rubbed their eyes. "No, I'm really just sleepy. I think I'm gonna go to bed now."

"All right, dear. Good night."

"'Night, mom."

"GOOD NIGHT, FRISK'S MOM!"

Papyrus, please keep quiet next time, okay?

"BUT-" Papyrus sighed, defeated. "OKAY."

Frisk found themselves in a dark hallway. It was hard to see due to the lack of reliable light sources or windows and the air was musty and stale, making it hard to breathe. Obviously this place hadn't been cleaned very often. But Frisk had other things to worry about, like the slow, soppy, squelching noise somewhere nearby, the whispers that seemed to come from all over the walls, the dread-filled feeling that something horrible was nearby.

Panic quickly overtook Frisk. They ran to a nearby door and flung it open, only to find another hallway behind it, with more doorways. There were stairs going up, stairs going down, but no exit anywhere. And the sounds and whispers only got louder no matter what direction Frisk took. They reached for another door, only for it to burst open before them. Frisk staggered back as a deformed, gangly creature that shouldn't exist lumbered out, dripping with some white-gray fluid, it's single, giant eye staring directly at Frisk, it's wings(...?) flapping erratically. No matter how much Frisk backed away, the thing always seemed to get closer. They cried out for help, but nobody came.

And then, as if it could get any worse, the thing opened it's beaked(...?) mouth and let out an uncontrollable flood of words, except they weren't really words, they were a maelstrom of random syllables, rapidly changing in pitch and overlapping each other constantly. Frisk closed their eyes and collapsed into a ball, covering their ears, but the thing's voice(s) grew louder still. After a while, Frisk could distinguish the owners of the voices. Papyrus, Alphys, Undyne, Sans. Frisk cried out for them to stop, for silence, for quiet, but no one heard them.

"frisk! hey, kid, wake up!"

Frisk gasped and sat up. Suddenly everything was quiet and they were back in their room. It took a few seconds of rapid breathing for them to get a hold of themselves.

...Sorry. I had a-

"a nightmare, yeah. we experienced it too."

What, really? The same one?

"i think so. take a listen."

Frisk didn't get what Sans wanted them to listen to at first, but now that they were paying attention, they heard a small sobbing noise, plus soothing, hushing sounds from a different voice. After a few seconds, they realized Alphys was crying, with Undyne trying to calm them down. Papyrus, meanwhile, had entirely withdrawn into himself and was completely unresponsive.

"I-I-I... F-Frisk, I... I-I'm s-s-so sorry I m-made you s-see that... I d-didn't know it w-would give you nnnnightmares... I, I should've..." Alphys blubbered.

"Sssssh. It's all right. It wasn't your fault. They're all with their families now, we made sure they were happy again. It's over." Undyne was trying her best to calm Alphys down, but it was obvious she, too, was shaken by the nightmare.

"Frisk? Are you okay?"

Frisk looked up to see their mom standing in the doorway. "Did you have a bad dream?"

Frisk just nodded, unable to think of anything else to say.

"Frisk, I'm worried. If something's wrong, you can tell me, whatever it is. Okay?"

"No, I... Nothing's wrong. It was just a nightmare, that's all."

Mom cast them a skeptical look. "Well, if you're sure. But if there is something after all, and you want to talk about it, then I'm always there."

Frisk hummed to convey they understood. Unfortunately, they couldn't take mom up on her offer even if they wanted to.

"Do you think you'll be able to go back to sleep?"

Frisk hmm-hmm'ed again.

"Okay. Good night."

"'Night."

The door closed, casting the room in darkness again.

"hey, frisk, i think we should talk about this dream of yours." Alphys had stopped crying by this point, so Sans was apparently confident enough to press the matter further. "i think you're afraid you'll turn out like that... thing. am i wrong?"

Frisk couldn't directly say yes or no. It was possible they were subconsciously scared that hearing voices all the time would drive them insane.

I... It's just... Well, the thought that more and more monsters are going to start talking in my head... There were thousands of monsters in the Underground. What if they just keep coming? I'll go completely mad.

Sans hummed appraisingly. "that's a very realistic fear to have. there's so many unknowns." Sans hesitated for a moment before continuing to speak. "alphys, you feeling better? think you can shine some light on the situation?"

"W-well..." Alphys' voice was still shaky, but then again, she never really seemed confident speaking. "I've been thinking about what exactly is going on. The most likely scenario I can think of is that when Frisk went back to their world, they took the souls of a bunch of monsters along with them."

"how much is a bunch?"

"No idea. Maybe just us. Maybe a dozen. Maybe even all of them."

Wait, really? So I have thousands of monster souls inside of me now?

"Y-yes, well, no, we don't know, really. We've got no baseline to compare it to. We already knew that it was possible for humans to absorb monster souls, but we have never seen it happen before due to the lack of longevity in monster souls after death. There's no telling what happens to the monster souls, o-or even how many souls you currently have in you."

You seem quite confident in this soul theory.

"It's the only thing that makes any sense. It would also explain why we have such difficulty moving by ourselves; monster souls are much weaker than human souls, a-and therefore performing the same actions takes much greater mental strain. A human body is attuned to a human soul, so it's simply too much for us to handle."

But what if I really did take all those thousands of monster souls with me? My brain can't handle that much activity going on!

"Good point." Alphys hesitated a bit before continuing with a wry tone of voice. "I never thought I'd say this, but the whole fiasco with the amalgamates now gives us some useful information. Thankfully, there seems to be a limit to the amount of souls that can be a-"active" inside a single being at any one time. That's why they clustered together in a number of entities, rather than one big clump. After a while, even their molten, sticky forms couldn't fuse any further."

Alphys paused again; now there was a melancholy note to her voice. "Miss Drake got the worst of it. She was merged with no less than sixteen vegetoids. Since vegetoid souls are comparatively weaker, she could more or less take "leadership". Unfortunately, their combined soul power wasn't enough to fully "nourish" their great body mass, a-and so they were left with a fleeting consciousness and unstable body. Even more so than the others."

"that's tragic and all, but how does that help us?" Sans was apparently getting impatient with the whole spiel, wanting to get to the point.

"W-well, Frisk only took the souls of monsters with them, not their bodies. Since their soul is so much stronger than ours, I believe it can more or less control what goes on in their brain. Not consciously so, of course, but I think it will automatically establish a limit to the amount of conscious beings. The rest of them will simply remain "dormant" until there's enough room for them to awaken. That's also why we showed up first rather than some random froggit; Frisk's soul is prioritizing the colleagues it knows and trusts."

Well, if that's the case, then that's reassuring, I suppose.

"rather. ready to try sleeping again?"

All right. Good night, everyone.

There was a chorus of mumbled responses, and then everything went quiet as Frisk entered a relatively dreamless sleep.

A/N: Whew, that was a lot of exposition. Hopefully now that I got that out of the way, I can move on to some more interesting situations.