I'm glad you all like my writing for this story. I'll try to make sure to not disappoint you guys.
If this author's note seem a bit weird, then it's because in sleep deprived :)
Enjoy the chapter!

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Phineas, Ferb, and Perry hide in an alley behind a dumpster and watch as several Normbot's hover by, looking for them. They leave when they find no trace of the boys.

“Oh, why do I have a bad feeling all of a sudden?” Phineas asked.

“Looks like they're gone.” They finally relax, but both Ferb and Perry seem to look expectedly at a Phineas, who looks oddly calm.

“You're a secret agent?!” Phineas suddenly burst out. “And you've been living with us this whole time?! Was that evil guy right? Were we just a cover story to you? I mean, were you ever really our pet or part of our family?” Perry looks down, depressed as Phineas continues to rant. “Well, apparently not, 'cause you didn't trust us enough to tell us. Anyone else around here leading a bizarre double life?” Phineas asks rhetorically. Ferb slowly raises his hand. “Put your hand down, Ferb.” Ferb lowers it. 

“You have a double life?” Candace asked. Ferb just blinked.

Perry seems almost to tears hearing Phineas rant.

“Now I feel really bad.” Ferb put his hand around Phineas’s shoulder when he saw how guilty his little brother was feeling.

“You're a secret agent! He's a secret agent!” Ferb puts a hand on Phineas' shoulder to calm him down. “You're right, Ferb. We gotta concentrate on the task at hand. We've got to get back to our dimension, and I don't even know where to start.” Ferb holds up the remote control.

“Oh, that's right, the remote!” Phineas said as he takes it. “I knew that would come in handy! Alright, let's go home!”

He aims the device and opens a portal. A pastel-colored forest area with a weird, six-eyed lizard-type creature is seen through the portal. “Wait, that's not our dimension.”

He shuts the portal, taps the remote, and opens and closes the portal in different spots, each having the same image.

“Nope. Uh-uh. Now this thing's broken.”

“What a piece of crap.”

“Holly!”

“What?”

“We're gonna need some help. I know! Let's go find us!”

They peek behind the dumpster to check for Normbots when Phineas suddenly looks at Perry.

“Wait. I just realized. You could've been cleaning your own litter box this whole time!” He accused.

“Hey, that’s right!” Candace yelled in realization.

Perry shrugs, looking a little embarrassed. “Oh, we are not done with this conversation!” he said, and they ran off.

“Let’s have that conversation with him after making sure he will still stay with you after all this.” Jeremy said, both to the onscreen Phineas and to his girlfriend.

Meanwhile, Doofenshmirtz is squeezed into a small school chair. Doofenshmirtz-2 is by a picture of Perry in pet mode.

“So tell me, other-dimension me. What do you see here?” Doofenshmirtz-2 asked Doofenshmirtz.

“An ordinary platypus."

Doofenshmirtz-2 flips the page, now showing an image of Perry in secret agent mode.

“And now what do you see?”

“Perry the Platypus?!” Doofenshmirtz gasps.

“You know, I'm starting to see why you haven't become ruler in your dimension.” Doofenshmirtz-2 said condescendingly.

“Wow. Even his alternate version thinks he’s dumb.” Holly said.

“Alright, Mr. "Eviler than Thou", just how did you manage to take over the Tri-State Area, anyway?”

“Simple. I used an army of big, scary robots.”

Doofenshmirtz-2 motions to a huge army of Normbots. They convert to the Norm heads. “We should do lunch sometime!” they said, simultaneously before they revert back to the armored heads.

“The Norm heads are scarier than the robot heads.“ Vanessa said with a shiver.

“Wow. That is scary.” Doofenshmirtz said, amazed. “I tried that robot thing once, too. I hid the self-destruct button on the bottoms of their feet, so nobody could reach them.”

Flashback to Doofenshmirtz with his army of bots, all of which have the foot with the self-destruct button raised up, him riding on the shoulder of one of the bots. Doofenshmirtz then calls out, "And march!", which causes the robots to take a step forward and explode.

Everyone facepalmed.

The scene shows back to the present where Doofenshmirtz looked quite foolish. “I think I've said enough. I still don't get it, if we're the same person, why are you so much better at being evil than me?” he asked.

“True evil is born through pain and loss.” Doofenshmirtz-2 said. “You see, when I was a small boy back in Gimmelshtump, I had a toy train.”

Flashback to young Doofenshmirtz-2, still with the eyepatch, but no scar. He holds a wooden train. Then it changed to him looking at the tracks.

“Then one day, I lost it.” Doofenshmirtz-2 narrated.

“Choo Choo?”

“Aww, I feel bad for him.” Buford said.

“Really? He loses a toy and turns evil. That makes you feel bad for him?” Baljeet asked.

“You don’t know the feeling of losing your precious toy!”

In the present, Doofenshmirtz stares at his counterpart. “That's... That's it?” he asked.

“What do you mean?”

“That's your emotionally scarring backstory? That's your great tragedy? Dude, I was raised by ocelots, literally. Disowned by my parents and raised by Central American wildcats, and you're telling me you lost a toy train? That's it? That's all you got? Really? I had to work as a lawn gnome, I was forced to wear hand-me-up girl's clothing, neither of my parents showed up for my birth!”

Everyone was surprised that Heinz had such a bad childhood. This man lived through all that and still lived a half-decent life?

“I don’t think I blame your father for being evil, Vanessa.” Phineas said, surprising everyone. “If I had to live a life like that then even I would have turned evil.”

“That’s not even half of the things that went wrong in his life.” Vanessa replied.

“Well, how did you feel when you lost that toy train?” Doofenshmirtz-2 asked back.

“Well, I never lost that toy train.”

“Well, maybe if you had, you would've done better.” He said. “Since you have neglected to take over YOUR Tri-State Area, I think I'll go over there and give it a shot myself.”

“Great, we can be a team!” Doofenshmirtz suggested happily.

“Yeah, right, a team.” Doofenshmirtz-2 said sarcastically.

“Wait, was that sarcasm?”

“Nooo...”

“Yeah, right there, I'm pretty sure that's my voice when I'm being sarcastic.”

“He’s going to use Dr. D, isn’t he?” Ginger said to Holly.

“Looks like it.”

In 1st Dimension, Candace, Stacy, Isabella, and Baljeet walk up to Buford, who is on a park bench, chewing some gum.

“Okay, Buford, where are Phineas and Ferb?” Candace asked.

“How should I know? They never showed up for me to return serve. That's considered a forfeit in catapult badminton, so I went to get some Victory Gum.” Buford said, showing a packet of gum.

~Victory Gum!~

“A tradition as old as the game itself.”

“So, a one day tradition then.” Lawrence said with an amused smile.

“Don't play with me, young man. Where is the giant platypult they built?” she demanded.

“I dunno. Seems to have vanished.”

“Oh, really? Did you hear that, Stacy? The mysterious force took the platypult away before I, the grown up, could see it.”

“Yeah yeah, I'm sure there's a logical explanation for all of this.”

Cut to the tow truck driver from before towing the Platypult.

“And they laughed at me for installing a platypult tow rig. Who's laughing now, Wayne?”

“The logical explanation is even more ridiculous than what I can think up.” Candace said with a laugh.

Back in the park...

"The logical explanation IS the mysterious force.” Candace said in frustration. “The real question is, why does it care so much about my little brothers? Why doesn't it want them to get busted?”

“Well, why don't you ask it, Kierkegaard?” Buford asked, and they stared. “What? Existentialist Trading Cards. It came with the gum.” He said, holding up a pack.

“Wait, he's right!” Candace shouted. “I should just ask it! I'll bet I can reason with it!”

“Reason with it? Candace, it's a force!... That you made up.” Stacy countered.

“No! I know what I'm talking about, Stacy. Come on!” Candace says, pulling Stacy with her.

“Where are we going?”

“To my backyard! The heart of the mysterious force!”

“Wouldn’t it be funny if there actually was a mysterious force?” Katie asked.

“More like annoying.” Candace countered.

The kids watch them go, and Baljeet pulls out two cards.

“Would you like to trade two Sartre for a Nietzsche?” He asked Buford.

“Alright.” Buford takes the cards and hands the other over, not realizing how valuable it is.

“Sucker!”

“Nietzsche is much rarer than Sartre.” Ginger said, causing everyone to look at her. “What? I like to collect it too."

“I think I’m in love.” Baljeet whispered to himself.

At the 2nd Dimension, a pair of Normbots hover by two trash cans. After they go, Phineas and Ferb poke their heads out of the trash cans.

“Is anyone even using those trash cans? They look too clean.” Gretchen asked.

“Well, this should be our street.” Phineas said, looking around. “But it sure looks different.” They pop out of the cans and run, Perry following.

2nd Dimension Lawrence is walking out of his house when Phineas and Ferb poke out of the bushes near the house.

“Dad?” Phineas asks.

“Boys, what are you doing here? You'd better get inside before the Doofbot catches you. I'm off to the factory. See you next week!” Lawrence-2 said hurriedly. He holds up a hook, which is attached to a harness he is wearing. A bus passes, hooking him on without stopping, along with several other people.

“No dad for a week! Man, I feel bad for the 2nd dimension us.” Phineas said, his siblings agreeing with him.

“It seems I work at some factory.” Lawrence said.

“Oh, you would not enjoy that at all.” Linda said, looking at the screen. “You always say you love working at the antique shop.”

“Wow. Well, I guess we'd better get inside.” Phineas is about to ring the doorbell when Perry walks up, and Phineas looks back at him. “You might want to... uh. I-I mean these guys might not know that you're... not really a... you know…”

Perry looks down and realizes that he’s still in secret agent mode. He smiles sheepishly, reverts to pet mode, and then chatters.

“He got used to being himself way too fast.” Candace said.

“Maybe he always wanted to be himself around you guys.” Stacy said, secretly trying to diffuse any situation concerning the platypus.

Phineas and Ferb stare at his seamless transformation, and an awkward pause comes. Phineas rings the bell, and they stare at Perry again for a while.

“Boys, I thought you were in your room. Get inside.” 2nd Dimension Linda, opening the door. “And get your Dooferalls back on before someone sees you. If you need me, I'll be hiding in the basement."

“That is so unlike you mom.” Candace said.

“That was weird.” Phineas said. “Let's find the other us.”

A hand is showing spin a Doofopoly spinner. It has only one no 1, and all others are 0s.

“And... hey, I got a one!” 2nd Dimension Phineas said.

“That game would take hours to complete, 1/3rd of that spinner are zeroes!”

“I think you meant 1/8ths, Buford.”

“Whatever! The point is that game is ridiculous.”

“Pick a Doofopoly instruction card.” He picks one and reads. “ ”Conform”. I can do that!”

“W-what? What is wrong with me in that dimension?”

“Hey guys!” Phineas said, walking in.

“Oh no, they're replacing us!” Phineas-2 hides behind the couch. “I must not have conformed quick enough!”

“No no, it's not like that. We're you guys from another dimension.”

“Different dimension? Is that allowed?”

“Apparently.”

“Oh look! Perry's back!” Phineas-2 runs up to Perry and hugs him. “Where have you been? We missed you so much!”

“How long do you think their Perry was turned to a cyborg?” Holly wondered.

“Perry would never stand by as Dad took over the Tri-State area. So other dimension Dad must have changed him since day 1.”

“Uh, Phineas,” Phineas said awkwardly. “he's not your Perry. He came with us.”

“Oh. Sorry.” he reaches to hand Perry back, then stops. “Wait, can I hold him a little longer?”

Phineas-2 hugs Perry again, and Phineas looks back at Ferb. Ferb and 2nd Dimension Ferb's eyes tear up, and they both pull out handkerchiefs for each other.

“It's just, well, he left one day and never came back. He's been gone for a long time and I'm really worried.” Phineas-2 said, handing Perry back.

“Looks like the other dimension you don’t know what happened to Perry after all.”

“Katie…”

“Sorry.”

“Alright, you know the only time we're allowed to make noise is on Doofens Day.” 2nd Dimension Candace said, coming into the room. “So keep it...” she suddenly seems to realize the room’s occupants and turns around. “Are there four of you in this room?”

“Five counting Perry.” Phineas stated.

“I see nothing. I have plausible deniability.” She said, walking away.

“Wow. A Candace that doesn’t bust.” Stacy said, surprised.

"A rare sight indeed." Jeremy agreed.

“Boy, your Candace is much less curious about what you're up to.” Phineas commented.

“What do you mean?”

“Well, it seems like our Candace has spent her entire summer focused on what we're doing.”

“Gee, I wonder why.” Candace said sarcastically.

“You didn’t have to bust them, though.” Milly said.

“And sit back and let them do their shenanigans? No way!”

“Summer? I think that was outlawed a long time ago.”

“WHAT?!” All the kids and teens yelled in surprise, except for Baljeet and Vanessa.

“You guys don't have summer?” Phineas asked, feeling sad. “Well that's-that's terrible.”

“Summer... It sounds dangerous, yet oddly compelling. What is it?” Phineas-2 curiously.

“What is summer? Man, where do I begin?” Phineas walked his counterpart back to the couch as the music started.

Background voice: ♪ Summer… ♪

“Why does think sound familiar?”

“Probably because you guys have already sung it once?”

Phineas: ♪ Summer, it's running through the sprinklers with your T-shirt, shoes and Jeans ♪

♪ Rolling, down a grassy hill yeah, that's what Summer means to me it's true. There's so much more to do. ♪

♪ The days are longer, The nights are shorter, The sun is shining ♪

Ferb: ♪ It's noticeably warmer ♪

Phineas: ♪ Summer, every single moment is worth its weight in gold♪

♪ Summer, it's like the world's best story and it's waiting to be told ♪

♪ It's ice cream cones and cherry soda dripping down your chin ♪

♪ It's summer, man, where do we begin? ♪

♪ Summer is ponds and pools and garden hoses, tryin' to beat the heat ♪

♪ Summer, bicycles and roller skates and even just bare feet ♪

♪ It's also... Surfing tidal waves, creating nanobots ♪

♪ Or locating Frankenstein's brain ♪

♪ Finding a dodo bird, painting a continent ♪

♪ Or driving our sister insane! ♪

“Oh, wait. Maybe we're going too fast.” Phineas said suddenly. Phineas-2 and Ferb-2 are seen shivering in fear.

“A terrified Phineas? That’s definitely a weird sight.”

♪ Summer, it's crickets and cicadas and a glass of lemonade ♪

♪ Summer, it's sitting with your brother in the backyard under the shade of a big tree♪

♪ That's what it means to me ♪

♪ The days are longer ♪

Both Phineas: ♪ The nights are shorter ♪

♪ The sun is shining. ♪

Ferb-2: ♪ It's noticeably warmer ♪

Phineas: ♪ Summer, every single moment is worth its weight in gold♪

♪ Summer, it's like the world's best story and it's waiting to be told ♪

♪ It's ice cream cones and cherry soda dripping down your chin ♪

♪ It's summer, man, where do we begin? ♪

♪ It's summer, man, where do we begin? ♪

♪ It's summer, man, where do we begin? ♪

Background Voice: ♪ Summer ♪

Phineas: ♪ Where do we begin? ♪

Background Voice: ♪ Summer ♪

Phineas: ♪ Where do we begin? ♪

“So that’s how we know that song!” Phineas realized.

“What do you mean?” Jeremy asked.

“We sometimes sing that song but we have no idea how we know that song at all. It’s good to know its origins.”