You're glad he's gone, and things are considerably easier for both you and your brother with him out of the house, although Papyrus keeps asking where he's gone.
But after a few months, you decide to go to the lab and ask for answers again, and maybe try to find Gaster. They won't let you in, and they say Gaster won't see you. You hang around near the entrance a few times to talk to him, but Gaster just ignores you and keeps walking.
Once, Gaster and Alphys come out together for a few minutes to talk, and they don't see you, so you quickly hide in a lava-formed rock formation above them and listen. You can see their troubled expressions from here since they're almost right below you, but you can't hear them very well. They both sound upset.
Alphys speaks too softly for you to hear, but you hear Gaster say, "I wish we'd never started this research. The king's anger could destroy us all. What exactly is the point of getting to the surface if we lose who we are in the process? You do remember the saying? 'Love, hope, compassion... This is what people say monster SOULS are made of.' If we act otherwise, and hurt people who never did us harm, it will create a conflict in our souls. That is what the queen attempted to tell him, but he refused to listen."
You hear someone coming from the left, and Gaster and Alphys stop talking and look in that direction. A large guard monster stomps by, his metal armor rattling loudly. Gaster rubs his hands together nervously, but other than that, Gaster and Alphys act innocent and nonchalant, and stay quiet until he's gone.
Gaster keeps rubbing his hands together agitatedly. Alphys tremblingly points to his hands, and says something anxiously. When he stops to reluctantly show her his hands, she gasps in shock. The bones in the center of each hand have been deeply scratched and damaged. Some of the scratches are recent, while others are scars. It probably happened over many days or weeks. You shudder at how bad it looks.
Gaster says, "Yes, I am aware that hurting oneself is a typical sign of mental distress, and I should stop. The fact that doing so makes me feel that I am reaping the just rewards of my actions, thereby temporarily relieving my distress, is not an excuse to engage in such a useless and counterproductive action." His tone is extremely cold, and you get the sense he deeply despises himself for it.
Alphys says something like, "... depression ... self-hatred ... cliff ... " which you only catch parts of, since she's speaking much more quietly than him.
Gaster listens soberly and then replies, "I may not understand your precise circumstances, but I believe I can relate to the frame of mind which you describe. If a person's actions conflict with who they are, it creates an endless, tormenting inner battle...I do not wish to hurt anyone. I never did. Yet here I am. Perhaps we have both disappointed ourselves, and now we are paying the price. The king relies on me so I cannot leave...and yet...sometimes I wish that someone or something would end me...I cannot..."
His tone changes to something more honest and real. "Alphys, I can't make the decision to stop, but I don't want to live and continue this either. I'm afraid of how my sons will look at me one day, if we don't stop. Maybe it'd be better if I died before they found out."
Alphys looks incredibly upset and worried, and by her tone she attempts to argue with him about it as they go back inside the lab.
You feel close to panic after hearing that conversation, and you keep trying to talk to Gaster but he always avoids you. Once you try actually blocking his exit from the lab until he's forced to talk to you, and he says in a chilly tone, "You are well aware this area is off limits to you, Sans. Please do not force me to have you escorted off the premises."
The other lab assistants look at you sympathetically but seem too scared to interfere. It bothers you how you can tell from their expressions that the easy going friendly atmosphere of the lab has disappeared, to be replaced with nervousness and hidden secrets. You give up for that day and leave, but you keep trying to talk to him, in places farther away from the lab that are free for everyone to be in, like Waterfall. He apparently really does live at the lab, but he still takes walks in Waterfall from time to time.
Once, when you're walking home again after trying to talk to Gaster again, on your way through Waterfall, you see a strange, not completely there door in the cave wall. You pause, unsure what to do, but you hear a muffled sound on the other side, like someone's in distress, so you decide to investigate. When you go through, you see Gaster crouched with his back towards you and his skull in both hands, his shoulders shaking. When he hears you enter he whirls around and stands up to face you.
"Sans!" he cries. His eye lights dart around like he's looking for an escape, but you're in front of the only entry way to this room. He forms some magic near one hand like he's considering making another door, but then lets it dissipate as he reconsiders. He stands there nervously, rubbing the center of one hand with the other, and it's then that you notice. Both of his hands have a deep hole in the middle of them, and scratches surround the holes.
You stop breathing for a few seconds in shock. It feels like your soul heart and the lines of magic through your bones clench for an instant.
You try to read his expression more carefully. You've rarely known him to be afraid of anything, but right now he looks beyond terrified, though he's trying hard to hide it. Underneath that, there's layers of guilt, shame, regret, self-hatred... and by the look of his hands, he's been taking it out on himself.
He tries to straighten up his tall frame and regain his dignity, and his expression settles into something blank and stony, but his eye lights show a true state of desperation.
"Tell me, Sans," he says, as if he's giving you a physics problem to solve like the old days. "Is there always a third way?"
You stare at him, unable to process the contrast between the lingering terror you're reading in his expression and the cool demeanor he's trying to portray.
"When you're trapped between two bad options, is there always a third way?" he asks you, stating it more clearly this time.
You think of Papyrus and his river puzzle. "yeah," you answer firmly. "even if it's not obvious, even if it doesn't seem that way at first, there's always another way."
"I see," he says, and his voice sounds partly cool and cold, and partly choking back a sob underneath.
"How is Papyrus?" he asks after a moment, and it sounds like an honest question.
"none of your business," you tell him sharply. You're surprised by the sudden rush of feelings of abandonment and betrayal in your soul. A part of you instantly regrets your words, but another part feels angry and protective.
His face falls and his shoulders slump. You take pity on him when you see it and decide to toss him a small bone. "he's coping."
His eye sockets close briefly and there's an expression of sheer relief that passes over him. "Good," is all he says.
"seems like more than you're doing," you say, with a nod and a glance at his hands.
"Just...experiments...I needed samples..." he says lamely, rubbing his hands together anxiously. He can tell you know it's a lie, a well-practiced excuse he's given others. He trails off, giving up on trying to pretend it isn't what it is.
"you know there's, uh, help you can get for mental problems like that, right?"
He shakes his head and asks dryly, "From who?"
"i don't know, you could talk to an echo flower? have it echo back self-affirmations?" You both snort at the ridiculousness of the idea.
"I think that would make me feel worse, to be honest."
"seriously, though, you need help. if you won't tell me what's really going on, at least talk to somebody. i'd rather not have to tell papyrus you um...you know..." You trail off, not wanting to say it out loud.
"He'd be sad, but it'd probably be a relief for you, right? I'm a threat to you, and him. I'm not safe. That's why you told me to leave."
"that's not what i'd call a helpful attitude, but in some ways, you're not wrong," you say gruffly. "you've been awful to us ever since you made me stop working with you at the lab. i don't want you to ever come back if you're going to treat us like that again, but that doesn't mean i don't care about you. i want to know why if i have to lose my own father. maybe there's still a good person inside of you, if you just try."
He evaluates you thoughtfully, struggling internally. You get the sense you're getting through to him.
"you're more than this, and you know it. don't give up."
He blinks, and his expression changes, as though he's temporarily pushed back a dark fog that was overwhelming him before. His eye lights look more focused like the fog has cleared from his vision, his body language looks more confident, and he reminds you of the old Gaster.
"I'm sorry, Sans. I had trouble gaining control. Thank you for reminding me who I really am."
You stare at him.
"the implications of what you just said are incredibly disturbing."
"I know. Please bear with me while I explain."
He talks quickly as though there's been something he's been wanting to tell you for a long time, and this is the first time he's had the chance.
"Sans, I have the feeling I might not have much time left. Something...something beyond, in the void, is haunting me. There's something dark out there. Something evil. Ever since we started the experiments with the void and the..." He shudders violently, and his bones rattle, as though he's been hit from the inside as punishment for trying to talk about the experiments. You tense up, alarmed by his strange behavior.
He steadies himself with an effort. "We should never have crossed that line and went against our morals. Everything's gone to hell since then. The truth is, going against my conscience made me weak, and it took advantage of that weakness. It's trying to make me do even worse things."
"why can't you just stop?" you ask softly. "tell the king no. do literally anything else?"
"Something's wrong with me. I'm not myself anymore. Sometimes I black out and the next thing I know, it's hours later and I can't remember anything that's happened. Sometimes it feels like I'm only half in control. It's taking more and more effort over time, to stay myself."
"can I check you?"
"Yes, if you want to."
You cautiously come closer to him and look over his entire body and soul. His health is less than half of what you know is normal for him. There's void energy mixed with his magic, and you can see it in the juncture points where his magic connects his bones. Some of it orbits around his soul.
You give him a questioning look. He shakes his head. "It's not just the void energy. The void itself is not necessarily evil. There's something in there...I can't make out what it is. But it's wrong. Very wrong. I can't get it out of me...it keeps pushing me to be someone I don't want to be."
You're no expert in psychology or a skeleton's equivalent of brain disorders inside the strong magical core of their skull, but you're pretty sure that intermittent amnesia is very bad sign. You wonder if it's true that something in the void is doing this to him, or if he's literally going insane. You resolve never to let him near Papyrus again as long as he's like this. His mental state is even worse than you thought.
"can't alphys help? she knows the most about this void stuff. you know, besides you."
"We're trying," he says. "Nothing's worked so far. That's why I've been wondering if I should just...never mind. I promised Alphys I wouldn't seriously consider that option again without talking to her first."
It's not hard for you to guess what that option is after seeing the state of his hands.
"can't i help?" You desperately want to help him, before things get any worse, but you feel weak. This is too much for you to deal with alone. But if he'll let you try, and stop pushing you away, maybe you can get others to help him.
"No," he says emphatically.
"why not?" you ask sadly, your hope for him ebbing away.
His white eye lights look into yours intently. "I want to make sure you stay out of this. I won't let you end up like me. Please...stay away from the lab and stop trying to talk to me. Keep Papyrus safe, like you've been doing. And Sans, if the worst happens..."
"please don't say that..."
"You have my consent to do whatever is necessary to stop me. I know I can trust your judgment."
"the hell are you talking about? ...are you saying you're giving me permission to...to..." You look at his expression. He's serious...
"shouldn't we just, uh, lock you up right now then, if you're not in control?"
"You could try," he says. "But if I tried to let you do that, I think...another part of me would object and attack you. Do you want to fight me?"
"no..."
He sighs heavily. "I almost wish that you had answered in the affirmative, except that I am uncertain if you could defeat me."
"i don't want to hurt you."
"I know," he answers. "I would rather die than hurt you or Papyrus. It would seem that we are at a stalemate. We do not wish to fight, but neither is the current situation tenable."
He glances towards the doorway restlessly.
"are you just going to leave again?" you ask, unable to keep a note of bitterness out of your voice. "there has to be a more creative solution than that. like you used to ask me to look for when i was stuck on a problem. isn't there anything more creative you can do besides hurt yourself?"
He half-grins ironically. "There probably is. I believe there is a way out of any quandary. I just haven't found it yet. This...thing...out there in the void...it's like nothing I've ever encountered. It's intelligent. It's vicious, in a way I've never seen or felt in any monster or human in our world. Don't underestimate it. Someday it might find a way to hurt more people than just by using me."
"that's why we should figure this out together. i'm willing to risk it, even if you're dangerous. can't you let me try?"
"I know you want to help, but I don't want to risk losing control around you," he explains. "I might switch back at any time. I don't want to become a horrible person again, while you're here. Believe me, I would greatly appreciate your assistance. Alphys is highly intelligent, but you are even more so. Perhaps it is my pride...I have already failed you so much. I don't want to hurt you even more by exposing you to danger. I want to try to solve this myself, somehow, before I even consider risking your safety by asking for your help."
"i think you're already way past the point where you should be asking for help, to be honest."
"Maybe. But I'm not done yet. If I fail, I've left notes for you in the lab so you'll know what I've learned so far. I've encoded the most important ones in our family's cypher fonts so the king can't order anyone else to use them. I trust you to keep your integrity and not make the same mistakes I did. I wouldn't trust anyone else with this."
Unbidden, a blue tear escapes your eye socket, and you wipe it away from your cheek bone with the back of your right hand.
He puts his arms around you and hugs you carefully and gently, and you let him. For a short time, Gaster is being completely himself again, like the father you knew. You wince as you see the deep scratches in his hands up close, and realize how strong his self-hatred must have been to make them.
"Sans, if we can't talk freely again, I want you to know that I am deeply sorry to have put such a terrible burden on you and your brother for so long. All of the fault is mine. You deserved far better than how I acted. Please let Papyrus know someday that the real me liked his river puzzle, and I kept his drawing. You're both completely amazing and incredible sons."
"why won't you at least try?" you ask brokenly.
"I am trying," he answers. "Sometimes you try your best and it's still not good enough. But that doesn't mean all my efforts are in vain. At least I'm keeping the two of you away from this. If I ever stop doing that, you'll know the real me is lost."
He lets you go and leaves you standing there, your eye sockets blurred with tears. By the time you recover, he's through the doorway and out of sight, and you rush around searching in a panic but you can't find him anywhere in Waterfall. When you return to that spot again later, the strange doorway has disappeared.