Okay, I don't think I really need to explain this one; I mean, I just put Phineas in Frisk's place because, one, he's my little marshmallow, and two, he has a striped shirt just like Frisk and all the kid monsters anyway. So it was only natural to pick someone like Phineas, even though Ferb would've been a good choice, too. Even though I think that Frisk kinda reminds me of a girl, I still think Phineas is the best and most obvious choice.

Take this into notice; I have NEVER played the game, ever. I've only watched let's plays, because my parents do not allow me to indulge myself in something like UnderTale for religious purposes. BUT THEY WILL NOT STOP ME FROM WRITING THIS FIC! XD Anyways, I'll try to be accurate, but this IS a Phineas and Ferb twist to it; I wanna make it kinda like an episode.

Note this, however, I will not be doing any yaoi or yuri shipping (NEVER AGAIN!), again, because of religious purposes; that's why Alphys is gonna be a dude in this fic…please don't kill me. It's just an AU. I can get away with small changes like that. O_O I do not judge anyone who ships Undyne and Alphys. I thought Alphys was a dude at first anyway, tbh, until I saw she had eyelashes.

Metaton stays the same, though…cuz…I have no idea what they are. And they're a robot/ghost, so I don't really judge 'em. XD

Phineas and Ferb are owned by Dan Povenmire and Jeff "Swampy" Marsh

Undertale is owned by Toby Fox©

So, without further ado, I introduce you, the PnF spin to UnderTale:

"SummerTale!"

Enjoy~! X3

P/F/P/F

Long ago, two races ruled over the Earth:

HUMANS and MONSTERS

One day war broke out between the two races

After a long battle, the humans were victorious

They sealed the monsters underground with a magic spell

Many years later…

(~)

Chapter 1: Phineas

MT. EBOTT

201X

Upon finally reaching the top, Phineas breathed in what he called pure, fresh air. The sun beat brightly down on him and his fellow companions, who were still following him up. He had been overly excited, of course. He looked back out at his hometown, the shining bright city of Danville, with a grin.

Seeing your hometown shine in the sun this way…fills you with determination.

Phineas nodded, yes, this was a perfect view. Another prefect start to a great adventure ahead of them. Phineas peered down the mountain, and called out, "C'mon, you guys, we're wasting daylight!"

Yes, he was most definitely far too excited for this.

At least, to Baljeet's interest. Speaking of him, he was miraculously the first to pop his head up over the mountain; although, this could be because he had been launched over by none other than Buford, a large, fat boy, who followed soon after. Baljeet flew over Phineas' head, and landed smack down on the dirt floor, with his mouth open. He gagged immediately, wiping the dirt and grass off of his mouth. Buford laughed boisterously at the nerdy child, while Phineas simply watched on obliviously to the "joke" before looking back to see if the others were anywhere near their destination.

"Augh…Phineas," Baljeet said in a small voice, "Are you sure this is such a good idea? I mean…don't you remember the legends-?"

Phineas was too distracted, of course, reaching out to another friend, Isabella, who nearly tripped up it seemed. "Gotcha!" Phineas grinned, "You okay?"

Isabella gratefully nodded at her 'rescuer'. "Yes," she answered, "Thank you so much, Phineas."

Phineas wasn't able to hear Ferb, his step-brother and the last to reach the top of the mountain, snort at Isabella's gratitude. Even though Phineas, oblivious as he was, wasn't able to really tell, Ferb knew very well that Isabella had 'tripped-up' on purpose to gain the attention of the boy of her dreams. Phineas did, however, notice the eye roll he gave, but didn't seem to detect the meaning.

Phineas, who seemed to be the leader of the group in spite his height and obvious immaturity, rubbed his hands together. "Alright," he said, "Is everyone here? Ferb?"

Ferb gave his brother a thumbs up.

"Isabella?"

Isabella raised her hand up, high and proud. "Present!"

"Baljeet?"

Baljeet's eyes shifted nervously; my, they were very high up, weren't they? "Uh…presently punctual…heh…"

"Buford?"

Buford pounded a fist onto his chest. "As if I would miss the many antics of Phineas and Ferb," he scoffed, snickering.

Phineas grinned at Buford's response in particular, clearing his throat. "Perry?"

Everyone paused to look around for their one missing companion. Phineas blinked. "Uh…say, where is Perry?" the boy wondered, "Ferb, I thought he was with you?"

Ferb looked around before giving a simple shrug. Isabella gave one as well, voicing, "Maybe he just didn't want to come along? I mean, I don't blame him; he's only a platypus, and this is Mt. Ebott."

Baljeet shuddered at the name of the said mountain. He looked to Phineas, who seemed to be nodding at what Isabella had told him. "Uh…why are we doing this again?" the nerd asked.

Phineas looked to Baljeet, very, very glad that he asked. "Adventure," Phineas stated, "We've lacked that in our lives for a while now, what with everyone being so protective. Especially our sister, Candace; I know it's terrifying, mysterious, and over all dangerous, but that's probably just what we need after…uh…certain events."

Buford and Isabella nodded vigorously at Phineas, both equally excited about the prospect. Phineas looked to Ferb, who simply blinked at him. Strange. Why did Ferb seem a little nervous? They were usually on the same page…

Baljeet rose and eyebrow, frowning deeply. "Phineas, please, do not tell me this is an act of rebellion against your older sibling," he said, rubbing his arm awkwardly.

Phineas frowned. "What?" he tilted his head, "Of course not! Why would that be? I just want to see if the legends are true. Everyone talks about it so much it was bound to be explored by someone."

"It has been explored," Baljeet said, "And…you remember what happened to them, right?"

Phineas sighed. Yes, he did remember. Who wouldn't know it? Everyone knew of this legend by now. It was practically taught in schools.

Phineas smiled calmly once again. "We're different," he stated, "We've done the impossible! We've survived a shattering city! We've explored unimaginable places! Heck, we've built a rollercoaster, time machine-"

"Alright, alright, I get it," Baljeet sighed, and added with a grumble, "I still wish you invited me to the time machine ploy…"

"Point is," Phineas stated, "We've done things no one else our age has ever done before! We can do pretty much anything we set our minds to; now, what do you say everyone? Whose with me?!"

"I am!" Isabella was the first to answer, practically singing as she slid over to the side of her future boyfriend.

"Me," Buford walked over to Phineas, nodding.

Against his better judgement, Ferb nodded his head as well, and put an arm around his brother, who returned the gesture with a large grin.

Everyone stared at Baljeet, who sighed. Baljeet extended his arms towards their destination; a large mouth to a mysterious looking cave. "Are we really going to risk our lives just to debunk a legend?!" he shrieked, "I do not think this is a good idea!"

"Welp, it's four out of five," Buford snickered, "So we outnumber your choice. Now, c'mon, nerd, we've gots some exploring to do."

Baljeet groaned as he proceeded to follow his friends into the cave, as slowly as he possibly could. He paused, and thought, wait, he could just turn back and climb down the mountain. It wouldn't be his fault if his friends-…wait…that's why he decided to come, wasn't it? Baljeet face-palmed at his own morality, and followed them into the cave.

The group with properly prepared, bringing bags of supplies with them. It seemed that their plan was to spend the night there like a camp out. Phineas and Ferb, to answer Baljeet's worries about the safety of such a feat, brought with them a pop-up tent of their own design, which was roomy, and could protect them from virtually anything. Everyone was carrying their own flashlight, looking around as they ventured forth.

"This is cool," Isabella grinned, taking out a handbook from what appeared to be thin air, "Stalagmite and stalactite, flowstone…this is one heck of a cave! I'll get my spelunking patch for sure!"

Phineas looked around, pointing his flashlight up. "Hm…no bats," he smiled, "Sorry Ferb. We brought the bat repellant for nothing. Hm…there seems to be a lot of plant life in here though…so many leaves and vines…"

Ferb, who had gone ahead of everyone, didn't seem to notice his brother's words. Phineas stared off after him, and soon he and the others managed to catch up to him. Phineas blinked at his brother. "Ferb?" he asked.

The green haired both pointed forward. "Look," he simply said, and everyone did as he said.

The group had immediately met up with a large, gaping hole, almost as big as the town square. Phineas, Ferb, and the rest looked on with surprised gazes, and looked up high and low. For such a dark area, it seemed to be light enough to see even without their flashlights, almost as if there was something trying to light up the room. Baljeet looked very nervous. "This looks…promising," he said, though obviously not meaning a thing; he was terrified, "I think we're done here…"

"Ah, c'mon, 'Jeet," Phineas grinned, walking over to the hole, "This is great!"

Isabella, Ferb, and Buford followed after Phineas, while Baljeet stared on in terror. "Are you kidding me?!" he shrieked in a hiss, "You do realize this is where people fell, right? This place it cursed!"

"Enchanted, Baljeet," Ferb corrected, making Phineas smile even brighter than before.

"Two lines in one day!" Phineas applauded his brother, "That's quite the talkativity, bro!"

Baljeet cringed. "Talkativity is not a word," he muttered before reluctantly following his friends over to the hole, "Look, I am just saying that we are going to be in so much trouble if-."

"Baljeet, Baljeet," Phineas shook his head, walking over to her nervous friend to pat him on the back, "We'll be careful. What's the worst that could happen?"

The sound of someone clearing their voice behind them answered Phineas' question, to which all the children froze. Slowly, but surely, the group turned to see the lovely, terrifying, Candace Flynn, who was staring at them menacingly as she crossed her arms, tapping her foot. Beljeet glared at Phineas flatly. "Told you…" he sighed.

Buford grumbled, the most disappointed of them all. "Great," he muttered, "Here comes the fun police…"

Candace gave a look that seemed wicked to any child. She pointed at them with a grin. "Aha!" she said, "I caught you red handed!"

Phineas paled. "Uh…hi…sis," he chuckled, "I know what it looks like, but-."

"I knew I'd find you here!" Candace stated, roughly poking her younger brother on the chest, "Mrs. Garcia-Sharpio said you were off playing adventurers again; said that you were 'pretending' to go to Mt. Ebott."

Phineas winced along with his friends, save for Baljeet, who was just glad that true disaster had been averted. Candace glowered darkly. "You guys are in so much trouble," she scowled, "Get your butts out of here before you get hurt!"

"Aw, c'mon, sis-!"

"No!" Candace crossed her arms sassily, "Absolutely not! Of all the places you brats could've been exploring, it had to be here?! Really?!"

Phineas stepped back, looking over to his friends for support. They didn't seem to have the strength to say anything, and shrunk back. Phineas looked over to Candace. "We were going to be careful," he assured his older sister, "We know how dangerous this is. We came prepared."

Ferb, Isabella, and Buford raised up their bag of supplies into the air. Buford elbowed Baljeet, who sighed and did the same. This only intensified Candace's rage, and the children stepped back once again. Candace rubbed her temples, sighing. "If mom and dad were here," she muttered more to herself than to anyone else, "You guys would be so busted…" then she grinned meanly, "Of course, I'm in charge now, aren't I? So, in fact, you guys are indeed, busted!"

Candace looked to Isabella, Baljeet, and Buford with a knowing smirk. "Oh-ho, I can't wait to see what your parents do to you," she snickered, then glared at her brothers, "As for you two-!"

Candace seemed to be rambling on about something, but Phineas didn't pay any mind. He instead decided to listen to a more, appealing voice that seemed to call to him. He closed his eyes in bliss. The voice sounded so nice…then his eyes opened.

Wait. What was it saying?

Phineas, without any warning, turned around, pushed passed his friends, and peered into the hole, much to Candace's annoyance and shock. "PHINEAS!" she shouted, stomping over to him, "WHAT ARE YOU DOING?!"

"I hear something…" Phineas squinted his eyes, "I think…I see something…it's a small light…maybe there really is something down there…"

Phineas became mesmerized by the idea that, perhaps, the legend was real after all. He grinned. "Hello~? Anyone down there~?" he called, making Candace and Baljeet shriek.

Candace grabbed Phineas by the collar of his shirt and pulled him back, glaring harshly. "I swear, if I didn't show up, you guys probably would've fallen down that hole," she said, "You idiots…we're going home, now!"

Phineas, to Candace's surprise, struggled in her grasp. She growled. "Phineas, what are you doing?!" she pulled on him, "C'mon, we're leaving!"

"I think someone's down there," Phineas said, "Maybe they need help?"

"I don't care," Candace scoffed, "They asked for it, I say! We're leaving, Phineas!"

Phineas didn't listen. He continued to struggle in his sister's grip as she tried to drag her brother away. Everyone else sadly, or in Baljeet's case gladly, followed, Buford still muttering that a good excursion had been wasted yet again by the ever annoying teenager. Yet Phineas, refused to give in. That voice…it was faint, but…familiar…

Finally, even before they could reach the path back, Phineas broke free of Candace's grip, and started to book it towards the hole. Candace followed immediately, with his friends also running after him, Isabella and Buford cheering their friend on, while Baljeet seemed to be scolding Phineas, and Ferb remained ever stoic to the situation.

"PHINEAS!"

Phineas only wanted to hear that voice, to make out what it said, but in his own hast he managed to snap his foot in one of the vines and tripped, heading straight into the hole…

…Or would've, if it were for Ferb, immediately sprinting over to rescue his brother. He managed to grab the younger boy by his legs, letting him dangle a bit. Phineas sighed in relief. "Thanks, Ferb," he chuckled again, "I…got carried away…"

"Yes," Ferb nodded, "Yes you did."

Just as Phineas was about to point out just how much Ferb was chatting away, suddenly a vacuum-like force seemed to activate, as it began sucking at Phineas and Ferb's grip. Ferb's body began to slide towards the hole, even as he tried to stand to resist it. Isabella and Candace went over to assist Ferb, followed by Baljeet and Buford, but the force of the hole was too great and their grip on the solid ground was too feeble.

"What's going on?!" Candace shrieked in panic, "Phineas and Ferb, what did you do?!"

"I-I don't know," Phineas said, "Keep holding on! Don't le-!"

Before Phineas could finish, Buford slipped, and everyone found themselves plummeting into the abyss, their screams echoing throughout the cave.

(~)

Legends say that those who enter the mountain never return…

[End of Chapter]

A/N:

Well, hope you guys enjoyed! X3 'Cause updates are, more than likely, going to be slow as usual. If any of you have ideas to speed up the process, I'll be glad to hear them.

I am so excited to see where this train-wreck goes!

-GTS