A Preface - During the prologue, the version of The Doctor we see is The Twelfth Doctor right after he lost all his memories of Clara, and right before River Song's death. When he was alone. Also, in this AU Gaster is a Boss Monster.
Earth, So Very Long Ago
"We're out of options, King Asgore. The humans are winning this war, and if things continue at the rate that it's currently progressing, they will drive us to extinction. The barrier device you asked me to build won't be ready for several months at the very least." The royal scientist Gaster stood before the King as he reported on his progress, the King silently considering this before responding.
"What would you have me do, Gaster?" He seemed to genuinely be at a loss for what to do as he asked Gaster his opinion, continuing to speak when the royal scientist failed to respond. "All I ever wanted, all any of us ever wanted, was peace. The humans started this war, and I knew from the beginning that we could never beat them in a direct war. The only option we ever had was that device and Project Core." He looked at Gaster expectantly as he said that.
"Well, in that matter anyway, I can report better findings. The early prototype for Project Core seems to be functioning as expected, and I've picked out an ideal location for the final version on Mt. Ebott, but again we simply lack the time to complete the project." The scientist readjusted his glasses as he spoke, reaching into his bag to pull out a handful of assorted papers covered in his findings which he handed to the King. "I spoke with Gerson on my way here, and he says that in this location we can only hold out for a week or two."
"Yes, I'm all too aware of that Gaster. I've spoken with Gerson and the other members of The Royal Guard, and all of our options are no good. If we hold out here, we can survive for no longer than a week. If we try to fall back, the humans could catch up to us and eliminate us in just under two weeks. The only option we really have is to try and fall back…leaving behind several members of the royal guard to delay the humans." King Asgore's voice was heavy with grief as he said that, clearly not wanting to go through with that plan. "Sylviana is currently trying to convince Toriel that we must go through with that course of action…yet I'm not even sure myself."
Gaster seemed at a loss for words as he put his research papers away, glancing towards the entrance to the tent as a member of The Royal Guard pushed an old man in through the entrance, ignoring his complaints. "Oh come on, the pushing is really unnecessary, I was only trying to help."
"King Asgore sir, we found this human out near the outskirts of the camp with a blue box." The royal guard member pushed him forward. "Do you want us to execute him?"
King Asgore was silent as he looked at the old man, who didn't seem to be very afraid. No, quite the contrary, he seemed more curious than anything else. That was what made his decision, in the end. "No."
"But sir-"
"I said no. If we start killing the humans' young and elderly, are we any better than them?" The old man seemed satisfied with this response, slipping free of his restraints as if they weren't even there.
"I was hoping you would say that. King Asgore, was it? I'm The Doctor, and if you'll tell your guard there to put down his weapon, I'll help you get out of this war without another casualty." The man smiled as he finished speaking, clasping his hands together behind his back, the guard's spear pressed against his back, ready to kill him if he reacted the wrong way.
King Asgore considered this in silence for a few more moments, before motioning for the guard to lower their weapon. "Alright, Doctor. You have my attention. I just have one question for you. Why would you choose us over your people?"
"My people?" The Doctor seemed amused by that, moving over to shake Gaster's hand. "It's a pleasure to meet you, really it is. You're so…unique. May I see that research you just put away?"
"Uhh…I suppose if it's okay with King Asgore…" Gaster pulled out the papers to hand them to The Doctor, watching as he flipped through the pages to look up with a grin.
"Oh, I like you. This research is absolutely genius." He handed the papers back to Gaster before turning to address the King once more. "First, they're not my people. I've protected the humans countless times and I will keep protecting them for the rest of my life, I'm sure, but sometimes that means I need to protect them from themselves. I won't allow them to wipe you out, as it would be a crime against both of your species."
He paused to consider something, before continuing a moment later. "Secondly, get your guards to escort all of your people to my blue box, there's plenty of room for everyone inside. Once everyone is inside, I'll get you all to that place your scientist mentioned, Mt. Ebott. Lastly, and perhaps most importantly, promise me no one else gets hurt. No other humans, no other monsters, from this point on no one gets hurt. I'm so very old and so very tired of watching people get hurt."
"I…" King Asgore hesitated to respond, not sure he could make a promise like that. He had to think of his people…which meant he needed to say whatever he had to if it meant The Doctor would help them. "I promise. No one else gets hurt."
"Excellent. I trust you'll keep that promise…because if you don't, I'll be…displeased. And you'll find that the universe is a very small place when I am angry with you." The Doctor said flatly, before he turned to face Gaster, satisfied with King Asgore's promise. "Now then, let's see if we can't perfect that barrier device of yours, science man."
King Asgore stepped out of the royal tent alongside his guards, motioning for them to do as The Doctor had said. "Get our people to this box of The Doctor's. We are out of time and out of options. Let us hope that this Doctor is who he says he is, because if he isn't…then we are all dead." The guards went about their duty to escort the innocent monsters to The Doctor's blue box as The Doctor and Gaster worked alongside one another to perfect the barrier device.