Once upon a time
There was a war between humans and monsters.
After a long battle, the humans were victorious. They trapped the monsters on a spaceship known as the skeld, that they could only leave once they finished their tasks.
Every year, a human would fly out and impersonate the other monsters in the skeld, trying to sabotage them. And every year, the impostor was spotted.
But one day:
Frisk flew their spaceship out to space. They narrowly dodged the asteroids plaguing the skies. Suddenly something was shooting out from the skeld, towards them. Had they already been spotted? Was their mission already over? No, it was aiming for the asteroids. They knew that their spaceship's cloaking design would not fail them. Blending in with the stars in the distance and the emptiness of space, frisk flew above the ship and landed on top. They pressed the self destruct button on their spaceship, this was a do or die mission. There was no going back. They hopped out of their tiny spaceship onto the outside of the skeld. They had heard that the others were spotted because they had a hard time blending in. Frisk had a better idea, simply remain hidden, no one has to know that they were there. They went in through the hidden entrance that the other impostors had been using for years. They found themselves to be somewhere with several beds, a computer, and what looked like some sort of scanning device. It had little red cross symbols on the walls. They had studied this map, and recognized that this room must be medbay. They realized that the doors were closed. Perfect. They were just about to use the vent in the corner when suddenly the doors opened and in came a woman in a purple spacesuit.
"Oh my!" She cried, "Yet another human child has come to the skeld!"
Frisk hovered over the kill button, knowing that this woman would surely rat them out to the rest of the crew.
"Do not worry! I am Toriel, the caretaker of medbay. I pass through this place every day to see if any humans have come once again." She said.
Frisk then decided that this woman could be of use to them, for now.
"When you are in Medbay, monsters may come in here to scan. They may suspect that you are a human, so you must try to blend in. Practice talking to this dummy."
Before frisk could ask, "What dummy?", Toriel had pulled out a life-sized crewmate dummy.
Frisk then entered into a fight with them, against their will. Their options had appeared below them, Kill, Sabotage, Report, and Mercy. Frisk laughed at the fourth option, knowing that this "mercy" would have no use to them. Frisk clicked kill and punched the dummy.
"Ah, the dummies are not for fighting, they are for talking! We do not want to hurt anybody, do we?" Toriel said, becoming concerned.
"It is getting late, isn't it? Now, why don't you go to bed." Toriel said.
Frisk headed toward the vent, ignoring her.
"Ah, ah, ah! We have no use of that! I think you should play up here instead." Toriel guided Frisk away from the vent.
Frisk tried to head back to it, but then Toriel put them in bed. Frisk fell asleep, unaware of how tired they were until now.
∾∾∾∾∾
They woke up. Toriel was sleeping in one of the other beds, across from them. It was still the middle of the night, and Frisk thought it was the perfect opportunity to escape. They were about to sneak out of bed when there was suddenly a knife floating in front of their face. A green ghost with a yellow stripe was in front of them. Frisk recognized her, the first human to ever be an impostor. Chara. She held out a knife and a heart-shaped locket. She was transparent and floating above the ground, which could only mean she was a ghost.
"Take them. The knife and the locket, and we will finally end the monsters." Chara said.
Frisk reached for them, and Chara disappeared.
∾∾∾∾∾
The lights turned back on. It was morning now, and Frisk's escape plan was ruined, but they were starting to form a new one. Toriel was awake and reading a book. Frisk came up to her and asked when they could leave.
"U-um," Toriel stuttered, "Would you like to hear about the book I am reading? It's about snails."
Toriel ignored Frisk's question. Frisk asked again, "When can I leave medbay?"
"D-did you know that snails can.... Stay here. I have something I have to do."
She closed her book and got up, heading towards the closed doors of medbay. Frisk followed her.
"Ahead of us is the exit of Medbay. I am going to destroy it. Every human that comes here meets the same fate. I have seen it again and again. They come. They leave. They die. You naive child... if you leave medbay. They... Asgore... will kill you. You want to leave so badly? Hmph. You are just like the others. There is only one solution to this. Prove yourself... Prove to me you are strong enough to survive!" Toriel blocks the way.
The four options appeared in front of Frisk: Kill, Sabotage, Report, and Mercy. It wasn't too hard for them to decide. They chose Kill and Chara's knife slew through Toriel like she was nothing. Toriel fell to her knees.
"Y... you... really hate me that much? Now I see who I was protecting by keeping you here. Not you... But them!" She said with the last of her will.
Only a purple corpse remained. Frisk vented away as if nothing had happened.
∾∾∾∾∾
Sans strolled by the closed door of Medbay. He had just come up with some new jokes that he thought were pretty killer.
He knocked on the door, "Knock, knock."
No response.
Maybe she didn't hear me, he thought.
He tried again, louder this time, "Knock, knock."
The doors of Medbay opened. Nothing could've prepared Sans for what he saw next. A dead body, bleeding over the white floors of medbay.
"No..." he whispered to himself.
Realization hit him hard. She was gone. And he never even knew her name. He hit the report button. Whoever had done this... was going to have a bad time.
∾∾∾∾∾
Everyone showed up in cafeteria. Sans didn't want to be the bearer of bad news, but it seemed like he didn't have a choice. Who knew when the impostor would strike next, or who else they would kill.
"So tell us, what exactly happened Sans?" Undyne said, who was not usually much of a detective, but was a fighter, and probably wanted to kill this impostor as soon as possible.
Sans would be surprised if she didn't immediately kill the first person someone said looked sus. And for that reason he wasn't going to be telling them everything, he had to have his secrets of course.
"I came to Medbay, the doors opened, and then i saw a dead body," he said.
Alphys stuttered, "W-who's B-b-body did you see?" the fact that someone died, making her very afraid.
"Truth is, i don't know her name. She was wearing a purple suit, and was always in Medbay with the doors closed." Sans said.
"Tori..." Asgore whispered, the weight of his dead ex-wife weighing heavily on him. Weren't the deaths of Chara and Asriel enough?
"It must have been an impostor!" Papyrus exclaimed, somehow too innocent to catch on to the fact that someone died.
"Now, I don't want to go pointing fingers," Mettaton said, lying to them, "but out of the seven of us here, and the other two that aren't here... UNDYNE SUS!!!"
"WHAT!!!" She exclaimed, hopping up on the table in rage.
"Well don't look at me like that, darling, it's not my fault you're always in weapons, I mean how sus is that? She probably has a secret stash of actual weapons in weapons!" Mettaton said, clearing trying to cause conflict.
"OH YEAH?!?!? How do we know you aren't the impostor since you are so eager to go pointing fingers!!!" Undyne backfired, enraged.
Mettaton scoffed, "ME? Darling, you must be out of your mind! I've been doing my tasks like a good little robot star!"
"Oh and I'm somehow sus? I'VE BEEN SERVING THIS SHIP AND OUR KING SINCE THE DAY I WAS BORN!!!!!" Undyne screamed.
Undyne and Mettaton started screaming back and forth, Papyrus started screaming too because he thought it was a game, Sans started to fall asleep, Asgore was thinking about Toriel, napstablook was just sitting there quietly, wishing he wasn't there, and Alphys was trying to stop the fighting.
"N-now, Everybody C-c-calm down!" Alphys stuttered.
But no one heard her and the fighting went on.
"M-maybe we can work this out?"
Still, no one had heard her.
She had had enough, "EVERYONE QUIET!" they all looked at her in astonishment.
"Uhhh..." Now that all eyes were on her she was starting to feel embarrassed, "L-lets not point fingers, okay? W-we should only a-accuse someone if we have proof..." She fiddled with her fingers and looked down.
"So its a draw then? We all won't vote and move on?" Sans was hoping this was the case, as he was starting to get a clue who the impostor was and he knew it wasn't anyone in this room.
"I don't know about that, Sans." Mettaton said, attempting to start an argument once more, "You're the only one who saw the body, how do we know you didn't self-report?"
"Oh please!" said Papyrus, "Sans can't be the impostor! He's too lazy. And he's been visiting that door for a long time, so even if he was the impostor, he would have... done whatever the impostor does... a while ago!"
"Fine..." said Mettaton, saddened at the lack of an ejection this round, "I guess its a draw..." He left the cafeteria, heading down to storage, Alphys followed behind them, and Napstablook just sort of... disappeared. Undyne and Asgore went to the left, and Sans and Papyrus to the right. Frisk had devised a perfect plan... and they were all falling into it.