A.N: MEGA SPOILER ALERT! If you haven't watched the new Phineas and Ferb episode "Happy Birthday, Isabella!" then you'll probably be really confused by where this story is picking up. I really don't want to spoil the episode for you, so PLEASE make sure you've seen it before reading this FanFic!

Also, a quick disclaimer: I do not own the rights to any of the characters, catchphrases, plot points, themes, or any other aspects of Phineas and Ferb. Please support the official show!

I'LL KEEP YOU MY FURRY LITTLE SECRET

Chapter 1: My BFF's Platypus Fights a Pharmacist

"You look kind of stressed, and you haven't even touched your pie," Candace observed, studying her friend Stacy carefully.

The two were sitting on the same sofa, but while Candace was slouched down and shoveling down a slice of apple pie contentedly, Stacy was sitting bolt upright and her eyes darted between her friend and the middle of the sofa, as though something were about to jump out of the upholstery at any moment.

"Is everything all right?" Candace pursued. "I'm your BFF, you know, best friend FOREVER! You can tell me anything!"

"Yeah, everything's fine. What are Phineas and Ferb up to?" Stacy replied, changing the subject a little too quickly.

"Phineas and Ferb!" Candace shouted, "Oh, no, I forgot! They've probably made an escalator to Venus by now! Or a foot-massaging cyborg cow!" She ran out of the room so fast that a dust cloud was left in her wake.

Anything, huh? Stacy wondered, smiling ruefully as she readjusted the pillows on the sofa to conceal a small black button she had spotted minutes ago. Would you believe the truth, even if I could tell you?

Stacy had been invited over with a text from Candace at 9:07 AM that morning. "Want to come over? Mom making pie…" The message ended with a sideways smiley face, an innocent touch that Stacy felt at odds with now.

A week ago I wouldn't have hesitated a second, Stacy had mused. But now…

It wasn't that Stacy felt any dislike toward her best friend; in fact, despite Candace's excessive sharing of Jeremy-related romance details and rants about "busting" her brothers, Phineas and Ferb, Stacy always treasured the time the two of them spent talking and hanging out. Sure, they had gone through rough points, but Stacy and Candace could never go more than a day after arguing without making up, and their friendship had always been the stronger for it.

No, it wasn't her friend that was Stacy's problem, it was actually her friend's—believe it or not—pet. A platypus, to be specific. For years, Stacy hadn't really given the teal duck-billed animal a second thought…until last Friday, when he came crashing into her living room on a careening truck, toting a fedora and locked in mortal combat with a pharmacist…no, wait, evil scientist. Yes, Perry, the Flynn-Fletchers' boring platypus was actually Agent P, a dynamic secret agent thwarting the schemes of the nefarious Dr. Doofenschmirtz on a day-to-day basis.

Considering Candace's brothers created inventions that defied the laws of physics every single afternoon, she really shouldn't have been that surprised, but for Stacy it was a shocking development. Only a plea to Perry's better nature and some quick thinking on her part had saved her from having her memory erased, and as part of the deal, Stacy had promised to keep the platypus's secret.

And so Stacy woke up the next morning and began her new double life. In theory, she reasoned, it shouldn't be a problem to go on as if nothing had happened; after all, the experience was a secret between only Perry and herself. In fact, she reasoned, it might be cool to have a secret agent for a friend, even if he did shed and eat a diet of worms and insect larvae.

However, she quickly realized that her newfound knowledge was a heavier burden than anticipated. It started off as small things; an accidental elbow into a trapdoor at the Flynn-Fletchers', a glimpse of Perry rushing off, the sight of the agent's paraglider sailing by, complete with logo (He has his own logo? she wondered at the time). As the days went on, the incidents got more noticeable, and she realized that the secret world that she had gotten a taste of was much closer than she imagined—in fact, it was omnipresent. Candace's house was riddled with secret entrances and exits; Perry's comings and goings were getting harder to ignore. One day she even thought she saw the Garcia-Shapiro's dog put on a fedora and disappear down a chute, although it happened so fast she couldn't be sure. And yet she was expected to keep the whole thing a secret? Was it even possible?

And now here she was, encouraging her best friend to indulge in her unhealthy brother-busting obsession just to dodge a question and rearrange pillows. As Stacy flopped back down on the sofa, trying to look relaxed and unaware of dark animal-related secrets, she wondered if maybe it wouldn't have been better to submit to a memory wipe after all. After all, did she really need to remember the ending of The Grievance?

But now it was too late; if she approached those guys from…eh…what was it again? "ORCA?" "OWCH?" She couldn't remember the name, but she was sure they wouldn't be happy that Perry had knowingly let her slip by without a memory wipe. Plus, now that the knowledge of Perry's identity had gone from her short-term to her long-term memory, there was probably a much bigger risk of an IQ drop from having that particular information erased. So she would just have to grin and bear it, at least until Candace and she went off to college and she didn't need to be around Perry anymore.

But can I really keep a secret from my best friend? The thought stabbed at her conscience like a poisoned dagger. Especially when it's about her? This goes against everything being best friends is about! How can I keep lying to her like this, even if it is to protect her and Perry?

Of course, it was at that moment that a teal, brick-shaped mass of fur plodded into the room. Speak of the devil…

"Oh, hey," Stacy said, rolling her eyes. "Just the platypus I was thinking about. Just so you know, you should really hide the entrance button in the couch more carefully." She smiled wryly, thinking of just how absurd the words coming out of her mouth would have seemed a week ago. "Anyway, are you planning on battling the forces of evil this afternoon?"

Perry remained unresponsive, looking for all the world like a complacent house pet.

"Look," she said, hunching over and giving Perry what she hoped was an encouraging look, "you don't have to play dumb when I'm the only one around. Candace and the boys are out in the yard, and…huh?"

Perry was still acting like a dumb animal, but his dull cross-eyed stare was momentarily interrupted by one quick glance up toward the ceiling. Not enough to be noticeable to anyone who wasn't watching closely, but enough to pique Stacy's curiosity. She followed the glance up to the far corner of the room, where…

"Cameras? You've GOT to be kidding me…" Sure enough, a tiny fisheye lens winked in the corner. She shuddered as it occurred to her that this was just one more thing she would have to try to ignore, one more secret separating her from the everyday life she had had a week ago. "Don't you give Candace any privacy? There are mikes too, I suppose?"

Perry started scratching himself animatedly with his hind leg like a dog, his head swaying back and forth. It took Stacy a couple of seconds to realize he was actually covertly shaking his head 'no.'

Stacy stared back at Perry and snorted. "Well, it's nice to see you people have SOME reservations."

Perry took another quick glance at the camera. Stacy averted her gaze, suddenly realizing that it might be bad to be caught on camera talking to an animal whose secret identity she wasn't supposed to know. She stared vacantly out the window instead, where Candace was shouting animatedly at Phineas and Ferb. Stacy was pretty sure she could make out the words "bust" and "going down" from Candace's lips.

Gazing at her friend through the window, Stacy suddenly felt very alone. A screen had been permanently cast between them, something that couldn't be fixed with a phone call or a trip to the mall. As things stood, Stacy could never be completely open with Candace again; now there was at least one secret she could never share. A part of her had hoped that she could lessen the burden by talking about the secret-agenty stuff with Perry (or, at least, talking TO him while Perry nodded understandingly), but now she realized that, with cameras watching every square inch of the house, even this small comfort was impossible. She would somehow have to bear the weight of it all herself.

"UGH, I DIDN'T ASK FOR THIS!" she growled, anger welling up inside her, then she realized she had voiced the outburst aloud. She glanced back at Perry, who was still watching silently.

"…Sorry, little guy" she said apologetically, feeling very awkward all of a sudden. What was the proper way to address your best friend's platypus, anyway? "I didn't mean to shout like that. …Anyway, why don't you go back to doing boring platypus things?" She glanced over her shoulder out the window. "Candace is coming back, so…"

With that, Perry plodded off in the direction of the kitchen, showing as much emotional response as the brick he resembled. As the platypus turned, Stacy caught a glimpse of a silver collar strapped across his neck, resplendent with circuits and digital readouts. Probably something Phineas and Ferb made, Stacy guessed, watching him crawl away.

"OOOH, THOSE BOYS!" Candace fumed, storming back into the living room. "Pheremone experiments! Disgusting bugs! Wait till I show Mom!" She stopped mid-rant, however, and stared quizzically at her ruminating friend.

"Hmm, sounds bustable," Stacy said, trying to sound interested.

"You bet it is!" roared Candace with righteous indignation, and for a moment, Stacy felt her chest swell with hope. Here she was like always, the levelheaded girl supporting her hyperactive friend from the sidelines. Maybe things were more complicated now, but that didn't mean she was any less of a best friend for it. What a relief…

Then Perry, who had been lying in the hallway to Candace's back, suddenly put on a fedora and began surreptitiously punching an access code into a panel concealed in the wall.

Ok, thought Stacy, forcing herself to take a deep breath, there's no big deal. All that matters is that I'm having a good time with Candace. Candace is looking in the opposite direction, so as long as she doesn't need to go into the kitchen-

Of course, it was at that moment that Candace's stomach decided to growl audibly.

"But before I bring the bust, I'm gonna need some crust!" shouted Candace cheerfully. "Time for some more pie, fresh out of the oven!" She picked up her empty plate from the coffee table and started to turn…

"WAIT!" blurted Stacy, loudly enough make Candace wince.

"Huh?" Candace said, surprised at the outburst. "Why?"

"I…er…um…" Stacy felt her heart rate skyrocket, trying hard not to look too visibly at the platypus disappearing into the wall behind her best friend. After a few seconds, the false wall slid back into place, and the coast was clear again.

"It's just…pie's…erm…" She had to think quickly. "…so FATTENING, you know! You gotta watch those calories for Jeremy!" She tried to hide her anxiety behind a smile.

"Stacy, are you sure you're ok?" Candace asked, real concern filling her eyes this time.

"What?" Stacy asked with a nervous giggle. "Of course! Yeah! Why would you think that? I've got nothing to worry about! We're just two BFF's hanging out, right? Best friends forever!"

But inside, Stacy thought: Friends forever, huh? I'm not so sure anymore…

END CHAPTER 1

A.N: Thanks for reading! I've published two one-shots before, but this is my first time attempting a serial publication, so please stay tuned for the rest of the story! I have the majority of the plot planned out, but I know that it's very easy to write yourself into a corner when you're doing something like this, so I'm a little nervous… My schedule right now is pretty busy, so I'm hoping I can write and upload the next chapter in a timely fashion.

Once again, thanks for reading, and make sure to check in for more!

-LiteraryDragon

P.S. It came to my attention that I've accidentally been misspelling "Stacy" as "Stacey." (This chapter is actually a repost with all the "Stacey"s corrected.) I'm really embarrassed! See, I have a childhood friend named "Stacey" whose spelling I have permanently engraved in my memory due to years and years of birthday party invitations, thank-you notes, and the like. It's really terrible to misspell the name of the main character in one's story, so please, if I slip again, just notify me and I'll correct the problem surreptitiously.