Papyrus burst through the front door to see Sans already melting into the carpet. His bones were completely unrecognizable. There was nothing left of his brother, not even an empty set of clothes.
With a breathy whimper, Papyrus collapsed to his knees next to the white puddle. Fumbling in his pocket, he pulled out the syringe of Determination and stared at it helplessly. Sans was already gone. There was nothing Papyrus could do to bring him back now.
At least he could join him.
With a second's more hesitation, Papyrus plunged the syringe into his own SOUL and delivered himself a massive overdose of DT. He felt the serum begin to work immediately, permeating his bones with an oddly familiar sensation, as though he were turning inside out. He shuddered as the burning spread through his entire body, his bones vibrating as they lost cohesion.
"S-sans..." he gasped, reaching towards the puddle with his melting hand. "I'm... s-sorry... I f-f-failed you..."
With a final anguished sigh, Papyrus collapsed. Within a matter of minutes, the only sign of what had happened was a large white stain across the carpet.
After hearing the bad news, it had taken Undyne several weeks to work up the nerve to visit Alphys again. When she did, she found that there was a lot more stilted chitchat and staring into teacups than actual conversation.
After a good five minutes of awkward silence, Alphys finally spoke up. Refusing to make eye contact, she mumbled, "I should have d-done something."
"No," Undyne responded reflexively. "We weren't there. There was nothing we could have done."
"J-judging by the debris left b-behind, they must have m-melted separately. If we'd kept a b-better eye on them..."
"We still would have lost them," Undyne finished the sentence. "Papyrus made his choice. He didn't want to stay behind without Sans."
Alphys fell silent again for a long moment. "The p-plan was a long shot anyway," she offered.
"Yeah," Undyne murmured. "It's a tragedy, but... it's hardly a surprise. Papyrus knew the risks."
"Still.... I'm g-going to miss them."
"I know. Me too."
After that, Undyne's visits to Alphys became more frequent. She threw herself into her Guard business, preferring the mindless thrill of fight training to the discomfort of being alone with her thoughts. It wasn't the same without Papyrus, though.
Alphys did the same, redoubling her research into improving the fate of the rest of the Amalgamates. Asgore allowed her to keep her position, but monitored her work more closely. Alphys was no longer allowed to conduct any experiments in secret. Undyne, as the Guard captain, took responsibility for overseeing Alphys's work in her free time.
Life went on in the Underground, as it always did. Sans and Papyrus's deaths, though tragic since skeleton monsters were now extinct, were a minor loss for all but their closest friends. After a few months, the carpet was replaced and the house was rented out to a new family.
The puzzles and traps that littered the woods slowly fell into disrepair. Over time, the resident monsters forgot why they had been there in the first place. Toriel and Frisk never left the Ruins again, and no one ever asked them to.
The world had moved on. Slowly, unwillingly, Undyne and Alphys moved on as well.
Undyne was half asleep, stretched out on Alphys's couch with one of her favorite anime playing on the TV. Normally Undyne tried to pay attention, at least for the violent ones, but she was wiped out from training and wasn't inclined to stay awake out of politeness.
She barely noticed when Alphys suddenly reached over and paused the TV.
"Mmm?" she mumbled sleepily. "Something up?"
"D-do you still miss them?" Alphys asked, without looking at Undyne.
Undyne squinted, slightly confused. "Who? Paps and Sans? Yeah, course I do. Why?"
"I d-don't know... it's coming up on t-two years now since they died, and I still think a-about them all the time." Alphys fidgeted, looking down at her claws.
Undyne yawned. "You still blame yourself, don't you?"
Alphys's silence was answer enough.
"Look, Al, I can tell you it wasn't your fault till the cows come home. But it's not going to change the fact that they're gone, and all we can do is keep going without them."
Alphys sighed. "I k-know, but..."
Undyne sat up halfway, leaning far enough to cover Alphys's claws with her webbed hand. "No buts, Al. We're going to move on, and we're going to do it together. Now turn the crappy show back on and let's watch it already."
With the show unpaused, Undyne fell asleep within the first five minutes. Leaning against her, Alphys fell asleep shortly afterward.
Life went on for everyone.