Chara: (blinks awake) Hm...?
* She seems to be trapped. Her ankles and stomach have chains around them, tying her to the wall, and her wrists are also cuffed to the wall.
Chara: Huh?! What the fuck?!
Adult!Frisk: Chara?!
* Frisk is right next to her, also chained to the wall in the same fashion.
Chara: Frisk!! W-where are we??
Adult!Frisk: I-it looks like the True Lab... why are we down here? Why did we pass out??
?????: You two are awake... thank god...
* Ben floats towards them. He has a weird metal collar around his neck, with flashing red and green lights on it. His hair is scruffy, and he doesn't look particularly optimistic.
Adult!Frisk: Ben!! What's that thing on your neck?!
Ben: (sighs) I can't say. It helps keep me in line until the deed is done, and it traps me down here until he takes it off. Say... no hard feelings on what's gonna happen... alright?
Chara: What do you mean?
Ben: I have a job to do. I'm sorry.
* He disappears.
Adult!Frisk: Chara... what's gonna happen to us...?
Chara: I dunno... but we're in this together.

ye no offence but chara isn't exactly the most healthy person.

Chara: r00d. Wait, who are you talking to?

* Papyrus walks down the hall, Ben at his heels, clutching his clipboard to his chest. Ben looks miserable, and Papyrus just looks like he wants to disappear...

paps no stop being cute i think i might be growing a soul

*sigh* why must you be so innocent...

Papyrus: (cute smile) Thank you, humans ❤️

you do realize your friends would be broken without you right? sans especially. you practically mean everything to him.

Papyrus: If I go out working on this, nobody will have to remember me. I'll die working towards a good cause, which is all I ever wanted.

This is going off of what BluTac said.
Papyrus, if you get killed, you're killing your brother with you. He puts you on such a high pedestal, that if he lost you for good, I'm honestly scared of what he would do. He's emotionally strong, but just one wrong thing could be the straw to break the camel's back.
If you won't stop for yourself, stop for him. I know you want everyone to be free, but there has got to be a better way to do it than risking your own life.
If anything, can you at least put the experiment aside for a bit and think about the consequences of what could happen to you and the others from what you're doing?

Papyrus: (breath gets shaky) I already thought about it. This is the safest course of action. I know Sans has ambitions of what to do when he gets to the surface. So does everybody. Chara wants to explore the forests near her home with Frisk. Prince Asriel and his family want to see the sun. And Undyne wants to see the water. I don't want to see them look up at the rock ceiling of the Underground anymore. If they feel bad for losing me... that's my fault for doing this experiment in the first place.

Let's put out a scenario.
You die or get erased, and the barrier is open.
Sans and Gaster get to see the real stars...
But... in that process, they would have lost the brightest star in their lives... and that star is you.

Papyrus: (eyes start to well up) Very poetic... but I'm not backing out of this plan. If there's the smallest chance the barrier can be broken without taking a single life, I'm chasing it.

Yet Chara was able to turn back. They were able to make amends with their family. Alphys, Frisk, Toriel, Asgore, Undyne, Chara, Asriel, they're all your family and while they do want to see the surface and be free, it isn't worth losing you. If you were able to remember your father after he fell into the CORE, there's a chance that they all might remember you. They won't be as happy without you. Please listen to us! We know what the surface is like.

Papyrus: But we don't. Chara may have been able to turn back, make amends with everyone, and live a happy life, but if everyone was to find out my plans for freedom, they would question if I was even sane...

But we won't be smiling. Does that mean that you don't see us as your friends? *has tears in their eyes*

Papyrus: Nonono, you don't understand! I do see you all as my friends, I really do! Every single one of you! I just... (sigh) ...it's hard to say. I'll say it another time.

I know it's not your intention to reset or possibly erase yourself but trust us! I promise in due time the barrier will be open and monsters will be free.

Papyrus: If we leave it up to that, we'll all be dead before the barrier breaks. I don't want to wait anymore!

Wait... WHAT

Papyrus: I made this choice a long time ago. I can't turn on myself now.
* He turns into the room where Chara and Frisk are tied, flipping on a smoke machine with a lightswitch to conceal himself from view. They don't get a chance to see his face before the smoke has filled the room.
Adult!Frisk: Huh?! Chara, I can't see anything! I can only see you!
Chara: There's smoke... I saw someone walk in... WHAT THE FUCK DO YOU WANT FROM ME AND MY FRIEND?!?!
Papyrus: ... (stays quiet in order to not revel his voice to them) (turns to Ben using sign language) "Now."
* Ben nods, and floats up to Chara, cupping his left hand around her soul and a needle in the other. He gives a bit of magic to the needle before positioning it at a good angle to Chara's soul.
Chara: BEN?!?!
Ben: Sorry... it's just business...
* He injects the needle into her soul, making a life force extraction. She tenses up from the sudden pain. Ben draws out the needle slowly, as to not hurt her any further, and holds it carefully. It now has a red substance in it, extracted from the soul. He heads to Frisk and does the same thing with another needle, before floating back through the smoke and handing the needles to Papyrus.
Papyrus: (sign language again) "Thank you for the help, Ben. Now remember, this is our secret, understand?"
Ben: Yes sir.
* Papyrus removes the collar from the ghost child's neck, then takes out the broken dog whistle, blowing on it loudly. The noise makes Chara and Frisk pass out again. He unattaches them from the wall, takes off his lab coat, then carries them all the way back to their room in the house and sets them down, ruffling their hair.
Papyrus: Sorry, you two... I promise this is the only time I'll have to do this.