The next day when Frisk came back, they were a little alarmed to see that Papyrus wasn't near Sans. Sans appeared to be asleep on the cot, while Papyrus was sitting at Gaster's desk, figuring out what 12 X 5 was.

Noting that the cell door was open, as usual, they started to step backwards out the lab door, trying to figure out if they had forgotten to call ahead of time, or something like that.

Too late, though, as Sans' head snapped up suddenly, looking straight at Frisk. Frisk stared back, meeting his eyes, one bright, the other cracked.

Then his lazy grin widened ever so slightly. "heya, kid."

At Sans' voice, both Papyrus and Gaster looked up at Frisk.

Papyrus grinned "Hello, Frisk!"

"Um-" They gestured slightly at the open cell door. "Um, is that..."

Sans sat up a little and waved a hand dismissively. "eh. 's fine. 'm not gonna kill ya."

They lifted an eyebrow, expression somewhere between curiosity and skepticism.

Papyrus nodded enthusiastically, though. "He's not going to! He likes you!"

Sans flopped back down on the cot, muttering into his pillow, "it's a little more complicated than that, pap."

Sans didn't continue, so Frisk looked to Gaster for an explanation.

He gestured slightly as he spoke. "You managed to do the same thing for Sans as Papyrus did. Not to the same extent, nowhere near, in fact, but you still did it."

Frisk blinked, then smiled. "So I helped to heal you, Sans?"

Sans, still facedown on the cot, just shrugged. Then, after a moment, he propped himself up on one elbow. "actually, it's kinda strange. i can still... feel you. i can feel your soul. but, lucky you, it's not driving me into a power-hungry rage."

"So wait. Does that mean you can... feel me? Like, where I am?"

Sans gave them a sort of unsettling grin and nodded.

At that moment, Gaster looked up from whatever he was working on at his desk, and looked at Sans. "Can you now? That's interesting."

Sans was grinning wide as usual, but his eyesockets went dark. "no. no. don't even think about it."

Gaster mimicked the disconcerting grin Sans had had, and innocently said, "Think about what, Sans?"

Sans just scowled at him.

Frisk, meanwhile, asked, "So, Sans, if you're not going to kill me now, can I ask you a question?"

Sans, who was getting kinda sore elbows by now from holding himself up at such an odd angle for so long, rolled over onto his back with a groan of effort. "i mean, you just did ask me a question. but i guess you can ask me another one, if ya really want to. like i said before, there's not a whole lot i can do ta stop ya."

"Can you still see out of your left eye?"

Sans was quiet a minute. When he spoke, he did so slowly, carefully picking over his words. "yes... but, if i pay attention, really pay attention... there are these dark lines over my vision, about where the cracks are." Despite his attempt to hide it, there was a clearly audible note of pain in his voice.

Frisk decided not to follow the next string of questions they had involving his left eye. Instead, they asked about his Magic, moving to sit against the wall of the cell. "So you have a bunch of Corrupt attacks, then?"

"yup." The relief in his voice was almost tangible. Any regret Frisk might've had about not asking about his eye instantly vanished.

"What's that like?"

"eh. nothing especially special. in the end, it all achieves the same thing."

Gaster snorted. "That's not what you told me." He glanced at Frisk. "When I asked him, he made it sound like going from Pure Magic to Corrupt Magic was like going from Windows Vista to Windows 10."

Before Frisk could say anything, Sans retorted with, "well, of course i'm gonna tell you it's a lot stronger! you're an enemy!"

Gaster rolled his eyes. "For the love of Annoying Dog, Sans, I'm not an enemy, I'm your father."

"who also happens to be on the opposite side as me in the war! you're pure, and i'm corrupt. which means we're enemies!"

"Except you're not Corrupt. Not fully, as you have tried to use as an excuse so many times before."

"hey! you're not allowed to say that! that's my argument!"

"How can it be 'your argument' if it contradicts what you're trying to argue?"

"because i came up with it, and i used it first! i just use it on other things."

"But it still doesn't work for you here, whereas it does for me."

"but if it doesn't make a difference in your experiments, then i don't see what difference it should make here."

"It makes a difference here because I'm the one who's deciding what does and doesn't make a difference. I am the rule of authority here."

In the month that Frisk had grown familiar with the lab, Gaster and Sans had not had one of their verbal battles. Whether it was just because there was less for them to argue about, or because Sans had been on good behavior to make sure he got to see Papyrus again, Frisk didn't know, but this was the first time they had heard their heated rapid-fire deliveries. They grinned as it went on, occasionally laughing at one of the impromptu arguments.

Papyrus came and sat down next to them. "See? I told you it was fun to listen to!"

They just smiled and nodded.

~o0o~

Eventually, Frisk realized what time it was, and said they should be getting home. Before they left, though, Papyrus came up with a brilliant idea.

"It's a Friday, and tomorrow we don't have school, so we should see if you can sleep over!" He then turned to face Gaster. "Hey, Dad! Can Frisk sleep over?"

Without looking up from what he was doing, Gaster shrugged. "I don't see why not, but you'll probably want to go home anyway to pack some stuff. Toothbrush and pajamas and whatnot."

From there, Frisk had called Toriel on their cellphone, who had agreed with Gaster, albeit in a much less careless tone. So Papyrus went home with Frisk, helped them pack up a few things in a backpack for the night, and then they both returned.

They spent a couple more hours in the lab, before Gaster said they had better be getting home. So they said goodnight to Sans, and returned to the Gaster family house.

After watching a movie, the pair had been getting ready for bed when Gaster declared in a very official, authoritative voice that it wasn't a proper sleepover without some good scary stories. So he got a flashlight, turned off all the lights in the house, and told them a story that had them up long after he had finished and gone to bed.

Despite the way they jumped at every noise, thinking it was one of the many horrors Gaster had described to them, coming to kill them, they eventually did sleep.

~o0o~

As it turned out, Gaster had actually done much better than he had expected with his story, and ended up being awake much later himself than he had intended. Which, never having been a man to set an alarm on a Saturday, meant he slept in much later than he had intended.

When he got up, Frisk and Papyrus were nowhere to be found.  Naturally, he assumed that they had gone off to play somewhere.

Gaster was barely finishing his breakfast when there was a knock on the door. He stood, walked into the hallway, and pulled it open.

Toriel was standing there. "Oh! Hello, Dr. Gaster. I came to pick up Frisk."

"Well, you won't find them here, your majesty. They went off to play somewhere."

"And... You did not ask where?"

He shrugged. "They left before I woke up. I don't think you have to worry, though. They're responsible kids, and Frisk did make it through the entire Underground, Corrupt and Pure territories, without dying. I'm sure their fine."

She frowned. "Still. I would like to know where they are, and Frisk and I have places to go."

"Well. They've got phones, don't they? We can call 'em."

Toriel nodded. That did make sense. She pulled out her cellphone and typed in Frisk'a number.

As the phone rang, Gaster's Soul skipped a beat. From farther down the hallway, he could hear a phone ringing.

Toriel heard it, too. "Let's not panic just yet. Try your son's phone." Gaster nodded and, after a moment of searching through his pockets for his phone, did as suggested.

Again, the ringing of a phone could be heard.

For a moment, they just stared at each other, the goat-bossmonster's eyes meeting the circles of light in the skeleton's eyesockets.

Then they both bolted down the hall, slamming into Papyrus's room. It was empty. Papyrus's phone was on the nightstand beside his bed, and Frisk's was still in their pants pocket, having been left there when they changed into pajamas.

Panic surged through the two parents. Their children were not there, and did not have their phones with them.

For both having experienced loss before, the idea of losing more was unthinkable.

Both started spewing off ideas of where they could be, what could have happened, but in that tense moment, both desperately avoided their deepest fear: a Corrupt infiltration and kidnapping.

Then a shard of a memory stabbed to the surface of Gaster's mind. Yesterday, Sans had said that he could feel where Frisk was. If they could find Frisk, using Sans, even if Papyrus wasn't with them, at least they would know what had happened.

He reached out and grabbed Toriel's shoulders. With no thoughts of the restrictions on the tenth floor of the lab, he said, "Come with me. I have an idea."

And then he started running.

~o0o~

Sans wasn't quite sure how he had ended up like this, but he couldn't say he especially minded.

He was sitting on the cot, back against the wall. Papyrus was leaning against his side, half asleep, and Frisk was leaning against Papyrus, fully asleep. They had come into the lab late last night, telling bits of a story, how some horrifying beast was coming to kill them. Sans had pieced together that Gaster had decided a sleepover needed a scary story, and then had proceeded to tell an exceptionally frightening one.

Of course, he couldn't just tell them to go back to bed, not when the only two people he remotely cared about were scared. And Papyrus knew the access code to the cell. So, he had invited them to come stay with him until they calmed down, to which they had readily agreed, and then fallen asleep.

Now it was almost noon. Sans had been awake for almost an hour, and Papyrus was just waking up.

And then the door slammed open, and Gaster and another monster Sans realized was Queen Toriel rushed into the room, in a panicked frenzy.

They both stopped at the cell and stared in. Gaster started to look annoyed at Sans, while Toriel just stared, shocked.

Papyrus and Frisk were both sitting up now, rubbing their eyes.

Sans saw Gaster's reproachful expression, and sighed. He knew Gaster didn't want Papyrus in the lab if he wasn't there. "aaaaand i'm gonna be in a lot a' trouble, aren't i?"

Before Gaster could respond, the queen spoke up. "How?" She demanded, turning to Gaster. "He's supposed to be dead."

The queen was probably the only authority figure Gaster had any respect for. Faced with said authority, any frustration with Sans of his slipped away to resurface later. "While I cannot answer how he is alive, I will tell you that I did not let anyone know he was alive because he is now also Corrupt."

Toriel saw the reason with Gaster's logic immediately, then shock registered on her face. "Corrupt?! Frisk, get out of there!"

Frisk, who was still groggy with sleep and not quite understanding the situation, groaned. "Hnnng... wha...?"

Sans, on the other hand, hopped off the cot and walked up to the glass wall of the cell, right in front of the queen. For a moment, they just stared at each other.

"If you hurt my child," the queen growled, a flame growing in her clenched fist, "you will regret it."

Sans just looked at her a moment. Then he shrugged. "okay. wasn't plannin' on hurting them anyway. not anymore."

Toriel blinked. That obviously wasn't what she was expecting. "What?"

Sans shrugged again. "they're an alright kid. i'm not gonna hurt 'em." And then, after a moment's hesitation, "... i promise."

Toriel turned to Gaster, seeking an explanation.

As Gaster got to work on that, Sans walked back over to the cot and offered Frisk his hand. "welp, kiddo, looks like you're safe from me. i don't like making promises, but i guess it's okay this once." He continued as Frisk accepted his hand and he pulled them to their feet. "anyway, your goat mom looks pretty mad. you'd better get goin'."

More awake now, they nodded. And, after quickly hugging first Papyrus, and then impulsively Sans, they slipped out of the cell to join their adoptive mother.

§

A/N

You know you're an Undertale Fan when your phone's autocorrect capitalizes Sans and Papyrus for you.

Anyway, thanks for reading this, and I hope you enjoyed it! And remember, votes and comments are not only always welcome, but always appreciated, too.