A/N: Hey, guys! Thought it would be helpful to lay down a few truths surrounding the story, just for some clarity moving forward. Stating them so plainly isn't exactly necessary to the plot and would honestly feel too inorganic to explain in the script itself, but they are something I keep mental note of while writing.

Frisk has never chosen the genocide path. In fact, it's never even occurred to them as an option. This is probably the most important thing to keep in mind for … reasons.

Frisk and Sans lived together on the surface for three years. This is why Frisk doesn't speak or think like an eleven-year-old, and why their relationship is so familial. (While I brought this up in Chapter 2, I thought it might be helpful to spell it out a little more plainly.)

I hope that adds some context! I wish I could just tell you everything (I'm practically bursting), but that wouldn't be fair. If there's anything you still wonder about, I'll be happy to clarify—assuming it isn't a spoiler, of course. :)

Hope you enjoy!

The deeper reaches of Waterfall smelled crisp and heavy with minerals like an old, abandoned mine shaft. Darkness shrouded every wall and offshoot like ebony curtains. Only the dim blacklight of stray mushrooms and distant blue rivers illuminated these caves. Even the crash of rushing water struggled to reach you, barely echoes off the walls. The sensation was akin to standing in a peaceful vacuum.

"uh-huh. uh-huh. okay, cool. sure." Sans leaned against the wall just outside Gerson's shop, phone to his head. He tapped his fingers in his coat pocket, absently watching you make friends at the riverside.

Save for the return of that throbbing headache, your injuries had recovered. It worried you, then, to notice that Sans only seemed worse. You had suspicions he had been healing you in your sleep. Regardless, his movements were slower, his quips not nearly as inspired. Though he insisted he had slept the past few nights, you found that hard to believe past the blue rings around his drooping eye sockets. His mind, too, had steadily wandered ever since mentioning that gray door.

He still wouldn't talk about it.

The air sang with white noise: waterfalls crashing, echo flowers murmuring, crickets chirping. Everything about this place made Sans want to float down and sleep. You were right, after all; he had starved himself of shuteye for days.

As he listened to his brother's monologue, his needs overruled his willpower. His eyelids started falling. His head dipped low on his shoulders. He nodded off.

"… AND THREE CUPS OF VINEGAR TO GIVE THE SAUCE A NICE TANG. NYEH-HEH! WELL, SANS? AM I NOT A GENIUS? SANS. SANS, ARE YOU EVEN LISTENING?"

His name burst through the speaker loudly enough to bounce around the cave. You turned just in time to see his eyes snap open and his shoulders sharpen. His phone spun so wildly out of his hand he barely managed to catch it in a cloud of blue magic. With a curse, he shakily returned the device to his tympanic cavity. After this many sleepless nights, the adrenaline tainted his magic like venom.

"yeah, i'm listenin'," he grumbled into the receiver. He stretched but cut the act short when the voice chattered more quietly. "i'm fine, bro. really. don' sweat it."

You sauntered over to him cheerfully despite your headache. Monster Kid never failed to flip even your worst day on its head. When Sans' eyes opened wide, however, you brought your good mood into question.

"oh. you did, huh?" He smiled at you strenuously. "heh. yeh … i'm sure they'll be great friends. okay. okay. okay. gotta go."

He hung up and grimaced at you plainly. You narrowed your eyes.

"He told Undyne where we are, didn't he?" you asked.

Sans mirrored Papyrus' usual galant stance: one hand on his hip, the other on his chest. "WORRY NOT!" he said in a surprisingly accurate imitation. "THE GREAT PAPYRUS HAS EVERYTHING UNDER CONTROL!"

Gerson guffawed around the corner.

"Last time he 'had it under control,' he told her exactly what I was wearing," you said.

Sans giggled. "yeah, he's a trip. but forgettaboutit. this ain't no seven bridges of königsberg. if it comes down to it, i can handle her."

At that, your bottom lip tucked away apprehensively. You had been avoiding Undyne for the better part of two days, now. Though you had certainly appreciated Sans' help dodging Mad Dummy's rockets and avoiding other monster attacks throughout Waterfall, the fish-like captain presented a more complicated challenge. You pinched Sans' sleeve and tugged him toward a more private nook.

"Sans," you said, "you can't help me on this one."

His smile drooped indignantly. "come again?"

"Undyne won't listen to words," you said. "You know her. No matter what any of us say, she's not going to back down through a conversation. She's all action. The only way to win her over is to show her I'm not here to fight."

As you were speaking, the hardened edges of his eyelights softened with understanding.

"kid, her teeth ain't even the sharpest thing about her."

"I know," you said, "but I've faced her enough times to know what to do. So, please … trust me on this?"

With hesitation, his squared shoulders slowly rounded. The tiniest twist of a smile slipped across his face. "trusted you this far, right?"

As you waded through the depths of Waterfall, Sans followed, occluded by a distant tapestry of tall grass and stalagmites. He had told himself to teleport away, to wait at his station on the edge of Hotland, but he could not leave you entirely to fate. His eyes watched carefully instead, darting between you and the phone screen in his hand.

Though set to silent, the application's colors and symbols shuddered as if disturbed. A thoughtful frown tugged at his jaw. Scanning the local rocks, foliage, and wildlife had delivered no clues. His fingers swiped and tapped to initiate an area-wide sweep instead. His battery would take a hit, but what the analysis lacked in detail, it made up for in a direction.

Just as he initiated the command, the foliage to his left crinkled like old paper. The steady rhythm of clashing metal stiffened his spine into a block of petrified wood. His head turned slowly left, and there he found himself face to chest with a tall, heavily armored figure. Though that polished pewter helmet hid her countenance from view, only one person could honorably wear this captain's armor of the royal guard.

Sans and Undyne regarded each other silently, breathing each other's air, breaching each other's aura. As a sentry, he had expected rage, disappointment, or maybe even hatred from his superior, but he sensed none of this. The way her shoulders spread firm like an iron beam intimidated even the strongest deities. When she observed him, however, her demeanor had become strangely resigned.

He knew, without a word, what she wanted from him. She wanted him to step aside.

She studied him a few dreadful seconds longer, then turned to the overpass, where you and that armless young monster now spoke.

Sans followed her gaze. He wondered how she could misunderstand your kindness. If actions mattered alone, hadn't your arms opened widely enough?

By the time he reeled in his gaze, she had already disappeared. Without her shadow looming over him, he realized how truly motionless he had stood. He released the air in his chest and struggled to catch up on what breaths he had left behind.

His phone buzzed: results cultivated. He had almost forgotten. As he scrolled through the mountains of information, his fingers stopped dead on a block of bright red symbols. His eyes became dark circles like new moons.

Even if short-lived, Monster Kid's decision to end your friendship always stung. In the future past, you had become close. You had attended school together, eaten lunch together, held sleepovers together. His saccharin heart beat too purely to despise anything, let alone another creature. So the moment Undyne showed across the bridge and MK stumbled to dangle by his teeth, you never had to fabricate your desire to save him. You threw yourself to your knees, reached out a hand …

The stomach-wrenching sensation of a shortcut swallowed you whole. Time and space distorted, and the fabric of reality peeled back to swap twenty meters for a single step. It chewed you up and spat you out onto damp earth, safe from the ledge and the abyss. You propped yourself up on your elbows. Your head roared in pain. You and your armless friend rested just before the bridge's end, and there, on the opposite side, stood Undyne. She tore off her helmet. You'd never seen such absolute fury.

"Sans!" she snarled.

You looked up. There he stood beside you, panting, brow knit in a mixture of urgency and dread.

"Hand over that human punk or go back to your station!" Undyne barked.

Sans slipped his hands into his pockets and stood like a locked gate at the foot of the bridge. His pupils never left her.

"make like a bullet, kiddo," he muttered over his shoulder.

Monster Kid wasted no time leaving, but you stayed put. A frown grew on your face. The implications of what he had just done buried thorns in the pit of your stomach, and you were unsure how long you could ignore them. You set your teeth. Your nails clawed into the ground.

"Sans," you began.

Undyne growled. A long streak of blue-green light sparked under her outstretched hand. She snagged what became a spear and targeted you before you could even finish his name.

Sans vanished in a splash of cyan, only to reappear at Undyne's shoulder. The moment she moved to throw her weapon, he shoved into her. Her magic smashed off-mark into the cave wall, only meters from where you knelt. Damp rock shattered into a cloud of debris and dropped into the chasm. After a heart-stopping delay, the sound of splashing rocks ricocheted back to you—fifty feet down at least.

You had to run.

Undyne's arm stretched back to take aim again. As you scrambled away from the bridge, several spears manifested behind her at once.

Sans must have interfered. No weapons hit their target. They collided with rock and crystal and showered you in particles like sand. Round after round of javelin throw came closer and closer to striking you. You braced yourself for each explosion, until a force exponentially more powerful than her spears forced you to kneel.

A flash of brilliant white cast an infinitely long shadow ahead of you. The dark streak lingered for less than a breath before succumbing to the light as well. The earth shivered. When you fought the reverberations to peer back, a bright beam had engulfed the cave. You blanched. A large, dragon-esque skull screamed white-hot energy out its unhinged jaws.

You had been right to be terrified of it.

By the time that skeletal apparition and its energy dispersed, the bridge had collapsed—and Sans and Undyne had vanished.

Far below the swinging remnants of a rope bridge, dark cave water danced to the rhythmless beat of rocks and planks. Without bioluminescent algae, only the soft glow of mushrooms illuminated this shadowy ravine. The waves glistened in the light, until slowly, everything settled into a glass-like plane.

It shattered again when an armored figure tore through the surface like Grendel from the swamp. Though she could breathe water, Undyne thrust the liquid from her lungs to draw air instead. A murderous growl slipped past the razors of her teeth. Her golden eye darted around the cavern until finding its mark: the skeleton materializing from blue magic at the bank.

"I should kill you," she spat.

Sans chuckled humorlessly and spread his arms. "not happy to sashimi?"

She cast a spear over his head. The limestone wall behind him flung gravel into the lake. He shoved his hands in his pockets, then. Darkness snuffed his eyes and his familiar grin spread very large, very taut, and very false.

Laden with heavy armor, dragging herself to shore took longer than she cared to admit. When she finally made it, she heaved Sans to eye level by his jacket collar. His eye sockets remained empty and his smile frozen.

"You're lucky Papyrus is your brother or you'd be dust right now, you traitor," she hissed into his face. "Did you forget about your fucking job?"

"i dunno what hot dogs got to do with this," he said coyly. "i mean hot cats , maybe …"

She flung him down onto his feet again. He staggered, snickering. As she observed the blatant exhaustion in his posture, however, her frown sourced from a different place in her heart.

"Go home, Sans," she said more calmly. She shifted in her armor uncomfortably. "Papyrus told me what happened."

Sans said nothing.

"You look rough, man," she went on. "Like, really rough. Like I don't even need to do a check to know something's up. You should be taking it easy right now, hanging at Grillby's, doing something chill, something other than committing treason for this human you barely even know!"

"did papyrus also tell ya that human saved my life?" he asked quietly.

Undyne didn't answer that.

He looked at her, then, left pupil dim but at least present. "that kid's the only reason i'm not blowin' in the wind right now."

"They're still a human."

"heh, yeah …" Sans looked away, smile slipping, "and i'm a skeleton. that's all that matters, right?"

Undyne's glare returned, though it diminished when she noticed a hard tear in the side of his coat. Small traces of dust caught the dim light, just enough to let her know that more than his clothes had been struck.

"Sans—"

"kid's waitin' for ya at the tunnel to hotland," he interrupted. He stepped midway into a shortcut and winked. "don' be late."

Just before Waterfall's last set of bridges, you skidded to a halt among echo flowers and dripping stalactites. Though a smattering of button mushrooms provided an ambient glow, this passageway stifled more light than any preceding. The warming atmosphere foretold your entry into Hotland—and your final conflict with Undyne.

You dropped your hands to your knees and gasped for breath. Pain insistently throbbed like a cat's kneading claws into your brain, even more harshly than before. Though confident Sans had given you ample time, you had been running nonstop. You had not risked one break, not until now, when the caverns failed to echo behind you.

Events finally caught up with you, and you glared at the rocks below your feet. Of all your Underground encounters, Undyne's was the most important to happen seamlessly. If she failed to see enough evidence to call you friend rather than foe, everything would fall apart. Sans had jeopardized it all, even after you had so plainly asked him not to interfere.

As if you had just drawn a pentagram in ketchup and chanted his name, Sans popped into existence right beside you.

"you okay?" he asked breathlessly. He snatched you by the shoulders and scoured you from head to toe. "those last few shots coulda scalped ya …"

You smacked his hands away. He buckled.

"Yeah, thanks a bunch, Sans. You really had my back, there."

For a moment, he was at a loss for words. It had been a long time since he had heard you snap at him that way. Even you couldn't remember having spat this much venom since the days leading up to your "fall." He seemed to understand, however, when his wide eyes softened to half-moons. The tensed corners of his mouth slackened.

"I asked you not to step in!" you said. "I asked you to trust me!"

"trust is supposed to be mutual, kid …"

"All you had to do was nothing." Drums pounded in your ears, drowning all else. "You're supposed to be good at that!"

He flinched. Though his hand twitched toward his heart, he forced it into his jacket pocket instead. His brow fell sharply, eyes razor-edged as if a retort brewed just behind them, but he bit his tongue instead.

"fine," he murmured. His dim eyes latched onto the floor. "i'll keep outta your way."

You were too angry to notice the emptiness in his voice. "Good."

At that, his frown deepened. He stepped painfully close until a breath separated your noses. The intimidation you had felt under his gaze in Snowdin crawled under your skin again, even more when his eyes darkened into shadowy pits.

"i get it," he said. "ya got yer system. but guess what, kiddo? that road you're after? those picture-perfect steps to the shiny diamond outcome you had before? it ain't happenin' that way, not this time. as hard as you might deny it or … tiptoe along your 'caution tape,' there's no telling if you're gonna roll sevens or snake eyes. if you wanna place your bets alone, fine … but whatever comes from it, that's on you."

His eyes relit, however dimly. Only disappointment shined there now.

Your anger slowly changed hands with regret.

"good thing you stopped resetting when you did," he said and vanished just as suddenly as he had returned.

Against no other sound, water dripped with cacophony. Not even your breath disturbed the air. After all, you weren't breathing.

What had overcome you? After what you had been through—after what he had been through—delicacy should have been in order. You had hardly given him a chance to speak let alone explain. Headache or not, you had never been that short-tempered in your life … right? The pain pulsated even harder.

When he said he would keep out of your way, did he mean … for good? Your stomach churned like a heavy-duty spin cycle. This guardianship was mutual, you remembered; you were supposed to be keeping an eye on him too. What if he hurt himself again? What if …

You wiped your face. No. Sans would not abandon you, not now, not ever. He was the closest thing you had to a father, to a brother, to family, and family never … Several human faces jumped to mind with the doubt of old ghosts. You hugged yourself tightly.

At his sentry station just inside Hotland, Sans sat alone. His phone lay cradled between his hands under the scrutiny of tired, tired eyes. Though he scoured Waterfall's scan results for the tiniest mistake, he hadn't misread the information.

Several points of temporal instability had come to light in the cavern, most notably the bridge. It had been shifting through different age states, whether that meant young or ancient or, more frightening still, entirely nonexistent. As far as he knew, the platform could have disappeared right from under you.

Still, he grimaced reproachfully. It did not change that he had ignored your one request. You probably would have been safe. The bridge's age states had been fluctuating so quickly that the milliseconds it disappeared were imperceptible. He would not have even known without the data in his hands.

As he watched the scenery dance in the molten air, he realized all at once how miserable he felt. Without you here to keep him busy, every sleepless hour, every crack in his soul, every self-deprecating thought, all piled over him. His ribs, too, seared with pain where one of Undyne's spears had sliced him just enough to matter. He dizzily pressed a hand to his side and huffed a small, dry laugh. He really must have been losing his grip if he were that easy to strike.

He struggled to preoccupy himself with charts and data, but focusing his eyes became increasingly difficult. Lightheadedness escalated until his vision of the Underground rocked like a ship on turbulent seas. Hotland seared brightly. He hid from the glare in his hood and cursed under his breath.

The sound of footsteps and clanking metal snuck up on him. He caught only a glimpse of a swift, blue and purple blur, quickly followed by a red one—Undyne hot on your trail. He smiled wryly. Good one. Guess your strategy had gone according to plan.

He closed his eyes, then, only briefly—or at least, that was what he had thought. When he opened them, the image of Hotland had disappeared, hidden behind a wall of unpolished wood paneling. It took him a moment to realize he had slid off his stool. His figure slumped in the corner of his sentry station, his energy sapped away, his phone well beyond reach. No choice remained but to rest here until you came back—if you came back.

His shaking hand lifted from the raw, reddened bone of his ribs and he rubbed his fingers together. Fresh dust ground between them like salt in powdered sugar.

unless i give out right here.

Your chest rose and fell in wild puffs to match the beat of your heart. Sweat soaked through the back and neck of your striped sweater. Why did you always wear it, knowing where you headed? Wool was far too warm for any exercise, let alone here on the edges of Hotland.

Undyne lay collapsed at your feet, steaming with heat exhaustion. If even a trace of humidity graced this arid wasteland, perhaps she would not be this overcome. Thank god for that conveniently placed water cooler to your right.

You pulled a paper cup from the dispenser and paused, dumbstruck. Had it been your imagination that the container … disappeared? Even if for a split second? You shook the feeling.

You poured the liquid on Undyne's head. She lifted herself to her feet as shakily as before, but this time, a deeper measure of understanding showed in the gold of her uncovered eye. She searched you longer than you remembered, then turned and walked away toward Waterfall.

Knowing everything had gone as planned relieved you, though you had noticed reluctance, maybe even uncertainty, in Undyne's attacks. What did she know that she hadn't before? Had Papyrus said something to her? Had Sans?

As you watched her armored back lose form in the swirling, sweltering air, your head pounded as viciously as before. No amount of sea tea or crab apples stifled the throb. Perhaps the cure lay with something beyond monster consumables. What you wouldn't give for some ibuprofen.

Just before you looked away, Undyne regained your attention when she stopped outside the sentry station.

This was new.

You narrowed your eyes, hoping to discern more through the swimming atmosphere.

Undyne reached inside the structure and lifted something carefully out. At first, the form escaped you. Then, the oil slick of the air cleared enough for you to realize that this something looked like someone, and just the right size and shape to be important to you. Your heart skipped out ahead. You sprinted across the precarious rock paths of Hotland, back the way you had come.

You paused outside the sentry post, breathless. Undyne had already disappeared into the dark of Waterfall, and there was no telling where she went from there. As Sans had explained more than once, this timeline was not the same.

A faint buzzing nabbed your attention. You looked left, then right, then peered into the guard post where those vibrations seemed loudest. There, abandoned on the ground like an old quiche, was Sans' phone.

The floor felt to drop out from under you. If his phone were here, then …

All your worst fears came to mind. Was he hurt? Had he hurt himself? Or had one more terrible thing come to pass, what Papyrus feared would send him over the edge into a final cloud of dust? You trembled. Had it been you? Were you the terrible thing?

You picked up the phone and winced at the caller ID. The guy in the photo was one pretty cool dude. His shirt even said so.

Your thumb hovered over the green button hesitantly, then swiped up to answer.

A/N: Oh boy! So much conflict this chapter. Frisk's attitude sure seems a little abrupt, doesn't it? HMMM.

Also, the Tumblr account (riftfic) is up to date! It comes with pictures.

Thank you for reading! If you have thoughts or feelings I love to hear them.

Next Time! What's up with this grey kid?